I've seen some games mange with a running "clock" and when you rest you choose how long you rest for, and in multi player you have to both choose the same amount of time in order to be able to accept the rest. This could also be neat, you carry around a bedroll with you, you can rest anywhere you want, (get in a short rest in a cave for example) but if you rest outside your camp you risk attack.
I think we'd be ok to not have the players in multiplayer need to choose the same amount of rest time since you can already explore relatively independently, maybe just make sure the party members that are chained to you auto unchain when you rest that way the other player can chain them if they want to wake up earlier.
Implementing a day/night cycle like this would end up affecting the long rest spams. currently the long rests do cycle a day through, so technically that could be used for quest time passage, but implementing a day/night mechanic and having the fast travel act like it did in BG1 where different travel jumps take a different amount of time, you would then have to choose "do I take 2 days worth of travel time just to go back to my main camp and lose 6 hours (4 for some) long resting then 2 more days of travel back here" or "do I take the risk of trying to short rest in this cave, maybe alternating watches but ultimately save myself a few days but I may be ambushed"
not suggesting sitting at a bedroll staring for 4 hours on watch either fyi. suggesting having an option available for "watch x hours" or "sleep x hours" and if player a choose to sleep 2 hours while player b chooses to watch 2 hours the game can roll through those two hours almost instantaneously (with background rolls of ambush chance/perception/etc....)
if player a chooses "watch 2 hours" while player b chooses "sleep 4 hours" it could skip through 2 hours worth of time then player a has 2 hours to do whatever while player b is still sleeping. which... that could get weird.... control another party member that's awake then and go do stuff???? idk actually....