i understand as a general rule you need to have something going on on both boards in order to put in any more money.

hypothetically, closing action, how many players need to be involved for you to stick around with a nutdraw for one board only?

suppose you have the nuts on one board, flush, straight or trips (not boat), how many players if any do you continue? under what conditions?

thanks,

Depends on a lot of factors, one of the most important being what the nuts are.

I am almost never folding if I have the nuts on one board unless it's a straight, or a set if there is very heavy action. If I am calling with just, for example, the nut flush draw on one board depends on a number of factors, but closing action it mostly depends on:
a) how big is the bet I need to call? the bigger the bet in relation to stack sizes, the more likely I am to fold
b) how many players are in the hand and what is my relative position to the player(s) doing the betting?
c) what players are in the hand and how likely are they to stick around if I make my hand?
d) what players are in the hand and how likely are they to fold if I make my hand and have nothing on the other board, ie will I be able to push them of second nuts type of hands on the other board?
e) what kind of backdoor equity do I have on the other board? if it's something like 944 I am more likely to fold (backdoor equity likely zero or close to zero)

Of course, this depends on what type of draw I have. With straight draws I am much more likely to fold since there is a good chance I am getting quartered and I would not continue without decent backdoor equity on the other board and only for a small bet. A nut flush draw on the other hand, especially multi-way, makes for a good candidate to continue with.

Here's where you can get yourself into trouble:

Suppose you flop the nut flush on top, which should be about 2/3 probability of holding up for the nuts. And suppose you have zero equity on the bottom. Let's say the preflop antes are $200 and you face a $200 bet.

You have at least 1/3 of the equity, so if the bet is all-in you should call. But if stacks are deep enough to play turn and river, there's a reasonable chance you might get freerolled on top. Turn and river bets will be much bigger, so you are protecting your 1/3 of the $200 antes by putting up $600 on the turn, assuming you still have the nuts, hoping to get that $600 back. And then if the nuts change on top on the river, you can face a tough decision in an $1800 pot -- all of this to defend $67!

So what if another player comes along? Now you're calling $200 out of $800 on the flop, so all-in that's a pretty easy call. But if stacks are deep, it's more likely you're fading viable draws on top with no equity on bottom. If the nuts change on top, your flush looks even worse against a bet and a call. And if the third player folds to a turn bet, on one hand that's another $200 dead so that's nice. OTOH you can now be freerolled for $800 instead of $600.

Anyway all this to say that yes, if the pot gets multiway you can justify some one-way calls, but be cognizant of the implications on the money left in the effective stack.