The plural form of 'attorney general'
The plural form of "attorney general" is less settled than fans of Eggs McMuffin might suppose. Here is the treatment of the plural issue in Merriam-Webster's Eleventh Collegiate Dictionary, eleventh edition (2003):
attorney general n, pl attorneys general or attorney generals ...
And here is the corresponding treatment in The American Heritage Dictionary of the English Language, fifth edition (2010):
attorney general n., pl. attorneys general or attorney generals ...
These entries indicate that a significant number of English speakers and writers adhere to one or the other of the two plural forms listed, but that the first listed form ("attorneys general") is the more common form. An Ngram chart comparing "attorneys general" (blue line) with "attorney generals" (red line) for the period 1780–2005 confirms a strong preference for "attorneys general" since about 1933:
I should perhaps note that it is extremely common in U.S. English to refer to "the state's attorney general" or (for example) "the Texas state attorney general" in all lowercase letters, which makes JeremyC's argument (in a separate answer) premised on treating the official title as a proper name unworkable, at least in the United States. Indeed, The Associated Press Style Book (2007) insists that even at the federal level—where there is only one attorney general at a time to worry about—"attorney general" should be initial-capped only when it appears immediately before a particular attorney general's name:
attorney general, attorneys general Never abbreviate. Capitalize only when used as a title before a name: Attorney General Alberto Gonzales.
The possessive of the plural form of 'attorney general'
In a May 13, 2013, posting on the Above the Law website, Bryan Garner—an expert on U.S. English usage and, more particularly, on U.S. legal usage—comes out four-square for "attorneys general" as the proper plural in the U.S. English; but then he endorses a classic editorial position on the question of how to express the possessive of the plural of "attorney general":
And how do you make the plural phrase attorneys general into a possessive? You don't, preferably. You might try to make a case for 35 attorneys general's briefs, but you'd induce more head-scratching than readerly agreement. To avoid any miscues, the better course is to rephrase with an of-genitive. So if you want to discuss the briefs of more than one attorney general, simply say the briefs of the attorneys general. Fortunately, most jurisdictions have only one attorney general at a time, so the plural-possessive form is not a problem you're likely to encounter often.
Clearly, Garner reckoned not with the rise in recent years of law suits filed by multiple states' attorneys general against the federal government to challenge everything from the legitimacy of the Affordable Care Act to (only a couple of days ago) a federal administrative rule change disallowing states from setting air quality standards for auto emissions that are more exacting than the federal standards.
Garner asserts British English normally uses the hyphenated singular form "attorney-general" and, therefore, prefer "attorney-generals" as the plural. The reality in British English seems to be considerably murkier than that. The Compact Edition of the Oxford English Dictionary (1971) offers a split recommendation:
Attorney-general. †1. gen. A legal representative or deputy acting under a general commission or 'power' of attorney, and representing his principal in all legal matters : opposed to attorney special or particular. Plural : attorneys general. Obs. [Citations omitted.] 2. spec. Attorney-General, Attorney General : a legal officer of the state empowered to act in all cases in which the state is a party. ... Plural (better) : Attorney-Generals.
So if you're talking about congeries of old-fashioned (or obsolete) all-purpose attorneys in the UK, the OED says that you should refer to them as "attorneys general"; but if you are talking about multiple states' legal officers, you would do "better" to refer to them as "Attorney-Generals."
But another British English dictionary, Longman Dictionary of Contemporary English, second edition (1987) expresses a different preference:
attorney general n attorneys general or attorney generals (usu. caps.) ...
Garner's assertion about the tendency in British English to hyphenate "attorney-general" notwithstanding, Longman lists only open forms and gives precedence to "attorneys general" over "attorney generals."
Garner's less disputable point is that the expression "35 attorneys' general briefs" is unlikely to pass muster even as a description of an underwear party. So realistically, you must either commit to "attorneys general's" or take the course that wise writers everywhere have taken.