Christine Ammer, The Facts on File Dictionary of Clichés, Second Edition (2006) essentially agrees with the assessment by John Ayto, cited in Mehper C. Palavuzlar’s answer. Here is Ammer’s entry:
two strings to one’s bow. More than one way of reaching one’s goal. This term comes from the custom of archers carrying a reserve string. It first appeared in English in the mid-fifteenth century, and by 1546 it was in John Heywood’s proverb collection. In the nineteenth century a number of novelists, including Jane Austen and Anthony Trollope, used the term as a metaphor for lovers: if one love affair fails, there is always another lover to be had. The current cliché is used more generally to mean resources in reserve.
I wonder, though, whether, both Ayto and Ammer may have missed something of the early sense of having two strings to one bow. Here is the occurrence in John Heywood, Proverbes (1546):
Yee have many strings to your bowe, for yee know,
Though I, having the bent of your uncles bow,
Can no way bring your bolt in the butte to stand ;
Yet have yee other markes to rove at hand.
I don’t know what to make of that passage beyond the possibility that the different strings may serve to shoot arrows at different marks. A footnote attached to the italicized saying in Julian Sharman’s 1874 edition of Heywood’s Proverbes cites a further occurrence of the saying in a June 1585 letter from Queen Elizabeth to James VI:
I am wel pleased to take any coulor to defend your honour, and hope that you wyl remember, that who seaketh two stringes to one bowe, the may shute strong but never strait ; and if you suppose that princes causes be vailed so covertly that no intelligence can bewraye them, deceave not your-selfe ; we old foxes can find shiftes to save ourselves by others malice and come by knowledge of greatest secreat, spetiallye if it touche our freholde.
Here the image is not of one working string and one string in reserve, but of two strings available for use simultaneously; Elizabeth seems to be using the strings as metaphors for human motives: A double motive may impel the arrow all the more strongly, but less straightly.
A Google Books search for variants of the phrase turned up a fair number of matches from the seventeenth century. I reproduce them here in hopes that they may provide a clearer idea of how the phrase was understood long before Jane Austen and Anthony Trollope arrived on the scene.
From Thomas Dekker, The Guls Horn-Booke : or Fashions to Please All Sorts of Guls (1609):
Thirdly (because I will have more then two strings to my bow) Comus, thou Clarke of Gluttonies kitchen, doe thou also bid me proface, and let me not rise from table, till I am perfect in all the general rules of Epicures and Cormorants. Fatten thou my braines, that I may feede others, and teach them both how to squat downe to their meat, and how to munch so like Loobies, that the wisest Solon in the world, shall not be able to take them for any other.
From a 1611 translation of Relations, of the Most Famous Kingdoms and Common-weales Thorough the World:
Where his [the Pope’s] authority is maintained, he hath supreme gouernment of all religious orders, and bestoweth the Ecclesiasticall benefices at his dispose. Hauing thus many strings to his Bow, he hath so many meanes to raise Money, that Xistus the fourth was wont to say, that the Popes should never want Coine, as long as their hands were able to hold a penny.
From Daniel Dyke, Michael and the Dragon, or, Christ Tempted, and Satan Foyled (1631):
Note wee here, the shamelessnesse of Satan in renuing his temptations. His mouth was stopped, and hee was set non plus in the former temptation, yet how soone doth he beginne to open his mouth againe? Hee was repulsed, yet he comes to fight againe. He hath many strings to his bow, and many arrowes in his quiuer. When one way takes not, hee tries forth with another ; yea, hee will make proofe of all ere hee leaues. He is called Beelzebub, the master flie. Flies though they be neuer so much beaten away, yet they will come againe and againe to the same place.
From “Part of a Letter from Mr. Robert Baillie to Mr William Spang, Minister of Campveer in Zealand” (April 25, 1645), in David Dalrymple, Memorials And Letters Relating to the History of Britain in The Reign of Charles the First (1766):
Our present posture here is this ; when the cunningness of Rothes had brought in Montrose to our party, his more than ordinary and evil pride made him very hard to be guided ; his first voyage to Aberdeen made him swallow the certain hopes of a Generalate over all our armies ; when that honour was put upon Leslie, he incontinent began to deal with the King, and when we were at Dunslaw had given assurance, and was in a fair way of performance, (had not the honesty and courage of Marshal prevented it) to have given over the whole north to the enemy. When our voyage to Newcastle came in hand, by his damnable bond, he thought to have sold us to the enemy ; thereafter he was ever in correspondence for our ruin ; Allaster Macdonald was the smallest string in his bow, and a design which he least trusted in ; but God resolving to humble us, who were beginning to swell with our great success in England, and on base partialities to be filled with emulations and factious heart-burnings, he would demean us with no more honourable rod ; some 1500 naked Scots Irishes having loppen from isle to isle, till at last getting away through Bdenoch, they broke down on Straithern.
From Henry Carey’s 1646 translation of Francis Biondi, The Second Part of the History of the Civill Warres of England Between the Two Houses of Lancaster and Yorke:
Notwithstanding all this Vanclere served and deceived them all, as the effects shewed. Hee demeaned himselfe in this manner, not out of any loyalty to the King, or love hee bore unto the Earle, but that hee might have two strings to his Bow, and doubly secure his owne interest.
From Clement Walker, “The History of Independency, with the Rise, Growth, and Practices of That Powerfull and Restless Faction,” in Relations and Observations, Historical and Politik, upon the Parliament begun Anno Dom. 1640 (1648):
This fellow Skippon was heretofore Waggoner in the Low-Countries to Sir Francis Vere, after that came over into England a poor forlorn Commander, and obtained of the King His Letters of Commendation to keep a kinde of Fencing School in the City Military yard, and teach the Citizens the postures of the Pike and Musket, and Train them ; where he wore the mask od Religion so handsomely, that he soon insinuated into their favours, and found them very bountifull Patrons to him, there he got his fat belly, and full purse ; from the City he became Major Generall to the new-modelled Army : and observing some discontent arising between the City and Army, and being willing to keep two strings to his bow, that he might uphold his credit with the City, he voluntarily submitted himself to some affronts purposely and politickly put upon him in the Army ; and yet that the Army might understand him to be their creature, he marched with the Army in their Triumph through the City, still carrying himself as a moderate reconciling man, and sweetening the intolerencies of the Army, by making milde and fair interpretations of their actions ; yet still so much magnified the power of the Army, as if he would perswade the City they were beholding to the Army for making no worse use of their strength against them :
from Robert Loveday, “Letter 123, to Mr. J.E.” in Loveday’s Letters Domestick and Forreign (1662, first published in 1659, after Loveday’s death):
I am still an Archer at Fortunes marks, and the other day narrowly mist the white of preferment : the best of it is, I have many strings to my Bow, and the Proverb says, The blind man sometimes hits a Crow ; but ad januam virtutis excubant labor & sudor [watch at the door of sweat and toil(?)]; and that’s the hubbe I aim at, I must ever scorn to physic my weak estate with gilded venome.
From George Etherege, The Comical Revenge; or, Love in a Tub (1664):
Sir Frederick Frolic. Pray, Widow, how long have you been/Acquainted with this Mirror of Knighthood?
Widow [Mrs. Rich]. Long enough you hear, Sir, to treat of Marriage.
Sir Frederick. What? You intend me for a reserve then?/You have two Strings to your Bow, Widow, I/Perceive your Cunning ; and Faith I think I shall do you/The heartier Service, if thou employ’st me by the by.
From Francis Kirkman, The English Rogue : Continued in the Life of Meriton Latroon, and Other Extravagants, Second Part (February 22, 1668):
And therefore taking a house and shop, that I might have two strings to my Bow, I again professed both Trades, of a Scrivener and Bookseller. I now thought my self wise enough to deal with the Booksellers ; but I soon found my self deceived, to my cost, for I was drawn in by some of that profession to be concern’d in printing of Play-books ; in which, I having skill, and much affection to the matter, willingly engaged.
From John Bunyan, “A Discourse upon the Pharisee and Publican” (1685):
So then, to seek for that that should save thee, neither at the Hands of the Law or the Hands of Mercy, is, to be sure, to seek it where it is not to be found ; for there is no medium betwixt the Righteousness of the Law, and the Mercy of God. Thou must have it either at the Door of the Law, or at the Door of Grace. But sayst thou, I am for having of it at the Hands of both. I will trust solely to neither. I love to have two Strings to my Bow. If one of them, as you think, can help me by it self, my reason tells me, that both can help me better. Therefore will I be righteous, and good, and will seek by my Goodness to be commended to the Mercy of God.
From Samuel Grascome and Samuel Hill, “The Second Letter” in “Two Letters Written to the Author of a Pamphlet Entituled, Solomon and Abiather : or, The Case of the Deprived Bishops and Clergy Discussed” (1692):
Having thus (as you vainly think) made sure of your inferences, you take leave of Civil, and enter upon Ecclesiastical Arguments. Now it is a sad thing, that a Man should have two Strings to his Bow, and both rotten ; for here you make more woful work than before ; the most specious of all your Arguments being either manifest Mistakes, or notorious Slanders.
The seventeenth-century instances cover a lot of ground and certainly include the sense that having two strings to a bow provides the opportunity to pursue one’s interests by alternative means. But I detect a pejorative tone in many (though not all) of these quotations: The archer with multiple strings to his bow seems to have them there at the ready so that he can abruptly change targets as the occasion demands or permits; the lurking notion is of expediency, possible double-dealing, and self-interest over any disinterested principle.
This older notion reminds me of the modern legal strategy of arguing in the alternative: I argue that my client was innocent of the killing and in fact was not even present at the scene of the crime; but if you find that he was present and did in fact slay the deceased person, then I argue that it was a case of self-defense, not murder or manslaughter. One bow, two strings.
It’s interesting that the modern interpretation of having multiple strings to a bow emphasizes having options in reserve and a kind of insurance through redundancy, so you won’t be defenseless if the worst happens and your primary bowstring breaks. Ultimately, how you understand the phrase depends on how you visualize a bow that has two (or many) strings.