The best way for co-op to work is to hand wave a lot of ingame time mechanics, use realtime for completing actions(a lot of stuff would have to be sped up, so more hand waving) and a tick-based turn system something akin to old MUDs.
It’d also never work for random online play. You’d strictly have to play with friends, and probably only one friend at a time. As for reality bubbles, just track the two for the players.
Also this post is not a “It’s so simple look at how it should work” statement. Doing the above is not simple, it’s just the best way I can think of how a multiplayer C:DDA would function.