Nothing much to add, mostly just confirming:
First choice of an onstage pair (with optional audience facing pair from stage) + SBD all recorded to the DR-680 using the venue's snake.
If that's out, use both of your recorders and..
Either expand on your known good setup without stressing out yourself and the venue-
AUD pair from your regular FOB spot, plus SBD to your M10.
(that gets you your standard AUD and lets you sync and matrix it with the SBD if you care to make the effort)
Or if it's cool to put mics onstage, but not use the snake-
Put the DR-680 onstage with a pair (or three or four) plus the optional aud facing pair. SBD to your M10 in back.
(that may get you everything you really need on the DR-680 alone, plus the SBD to sync and matrix with it if necessary)
Substitute your friend's DR-680 for the M10 back at the board if you like. Oh, and eventhough Scoob mentions shotguns or supercards for the optional audience facing mics from the stage, that pair can be useful regardless of mic pattern, so if you have an extra pair of cardioids or omnis and the time to set them up without stressing things, go ahead and use them. You don't have to use those channels but may as well run 'em if you got 'em.