Look up how to synchronize using MTC - instead of MMC.
It's typical to use MTC to lock a pair of VS recorders together.
One is set as master and controls start/stop/rec functions on both machines, when operated from the MASTER transport controls.

(Don't operate the slave transport controls separately, without disengaging MIDI sync, or it messes with the sync.)

Pg 165 of the 840gx pdf manual and pg 116 of the pdf 880 manual should be the MIDI sync sections... or thereabouts.

It's fairly painless for simply syncing the 2 machines and recording. But I strongly advise that you spend an hour or two - not at the gig - learning how this works. It can seem confusing and it has little quirks (like being sure to allow for a few seconds of "pre-roll" while the two machines lock in place), but it's totally doable.

Monitoring the audio takes some figuring out, if you plan on listening through speakers (personally, I wouldn't bother during the live recording and just use headphones, switching between machines to check as you need to) and playing back in between recording takes briefly messes up the midi sync - make sure you set TAP markers at the end of each recording take, so you can quickly jump back to where you left off, if you should want to check the playback between recording takes. And then remember that midi sync takes a few heartbeats to lock, when you start recording again, so don't expect to hit record right on a count-in or something.

Best advice is to experiment for a couple hours, putting it through its paces and you will quickly get the hang of it and be ready for the live recording without having to sweat about it so much.