[cheburashka]

OK, I'll contribute in the interest of discussion, even though I have no idea what I'm doing.

First, what might the villains have opened with?

If SB is 50BB deep, and he flat called rather than raised--he's not desperate to get it in, and he's not anxious to get you out. To me, that says he has a pretty good hand he wants to play, probably one that needs improvement. I would exclude lower SCs, as they suck to play OOP, and low PPs, because you're too short to pay off a set. Let's say 77-TT, A9s-AJs, ATo-AJo, KQ, QJs, JTs, T9s.

BB is looser, and he's getting almost 4:1, but he's still deep, and he must have some respect for a UTG raiser and a decent caller. So balancing it all out, let's give him the same range as SB.

Second, what do you need to C-Bet? I'm going to assume that you will get out if you are check-raised, and if you are flat called, you will be very unhappy if the turn is an 8 or higher (and reasonably unhappy if it's anything else other than a 5). So pretty much you have to take the pot now or it's gone. Standard C-Bets at 5.5/11 tend to average 2/3 of the pot, so you'd need both villains to fold 40% of the time to break even. That means your C-Bet would need to get each villain to fold ~60% of their hands, taking into account that they will be getting 1.5:1.

Of the range I've assigned the villains, you could probably A9-Ts, ATo, KQs, T9s, and maybe 88. The math here is far beyond my simple mind, but it doesn't look like a C-Bet makes sense if this analysis makes sense.

Of course, in the heat of the moment, I'd have to rely on rules of thumb, and one that I have is not to C-Bet into more than one player if there is a T or higher on the board, unless I have a really good read that tells me otherwise.