I think you are misunderstanding the choices being presented.
It's choosing between:
1. Working on both in somewhat parallel, (e.g. a few hours of MNF until the dev gets a bit blocked, or comes up with an idea for MNM, or simply finishes a day/event and comes to a good breaking point then switches over to working a bit on MNM.
2. ONLY working on MNF until the writer finishes a full update, then switch over to MNM until finishing the full update, etc.
Either way they are both (theoretically) progressing. 2 seems like it may be better for Patreons, to let them speak with their wallet as to which game they prefer the dev spends time on, (by changing tiers from month to month) and (potentially) creating more accountability for frequency of updates. 1 seems more flexible allowing the writer to respond to flashes of inspiration when they happen... but maybe the dev actually LIKES being "siloed", and focusing exclusively on one thing at a time... though, 1 doesn't really prevent the dev from self siloing if he's really on a roll with one of them.
I'm not a patreon, so don't particularly care. I happily leave it with whatever workflow is better for the Dev. (or, I guess, whatever the Patreons choose for him.)
P.S. If the dev has a preference, feel free to post it, and give us "whatever the dev wants" a say, I guess. That, or make it a 3rd option in the poll.