And they fell to upon their frugal supper.
(From Vanity Fair)
I can only find either to fall to, or to fall on / upon, but never to fall to on / upon.
It is a literary idiomatic expression, often used referring to eating:
Energetically begin an activity, set to work, as in As soon as they had the right tools, they fell to work on the house.
This expression is also often used to mean "begin to eat." Charles Dickens so used it in American Notes (1842): "We fall-to upon these dainties." [Late 1500s]
- So we quarrelled, and by and by we fell to upon that fight in which thou did see us engaged."
- I have already mentioned, and so fell-to upon a very hearty supper; but, as we ate, each man had his weapon stuck in the sand beside him; for we had knowledge that the valley held some devilish thing....
More usage examples are available in Google Books.