What does "to fall to upon" mean here?

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:

Fall to (on/upon):

• 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]

(AHD)

Usage Examples

From: Robin Hood/King Arthur's Knights:

• So we quarrelled, and by and by we fell to upon that fight in which thou did see us engaged."

From Sea-cursed:

• 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.