Live tournament had a weird situation.
Player A having 50+BB raises 2,5x, Player B with similar stack calls, now player C pushes allin for his 6BB.
Player A says call, player B immediatly also says call, now for some reason player A opens his cards and so does player B allthough they both should continue to play.

Whats the ruling here?

I could be wrong but I'll take a guess.

Both players will probably receive a penalty after the hand but both will still be able to play their hand which means one will probably iso all in on flop (probably whichever has the best equity on flop out of the two). To the all-in player it's kind of a good thing but also kind of bad. Good because one player should iso flop and he'll only have to beat one hand from there on. Bad because someone with say 9 high probably won't iso where in a normal situation they might with a semi-bluff. Even still though say the two face up hands are A3 and KQ on 789 and A3 iso's, if the all in player has TT he's only fading 1 over card now. I think it'd work in his favor more than not and can't see any way this will work strongly against him so don't see any reason a TD would not allow the two face-up players to act. If only one player turns their cards up the rest of the players get to play the rest of the hand out as normal (as does the face-up one) so when two players exposed their hand I can't see why this would change.

Pretty straightforward... they continue the hand now knowing what each other is holding.

I will agree with this and add that most likely all they will get is a warning about exposing their hand as this seems very much like it was unintentional. If I felt at all as though they were colluding to beat out the short stack, I would not hesitate to give a penalty.

How might exposing your hands help here? Serious question, I thought about it and have no idea how that could give them any advantage over the standard (and generally accepted) implicit collusion of snap check-checking it down.

I agree with you that it would not help them in any way, but I was thinking along the lines of if they had openly made the agreement to check it down or something and then just flipped em over to run it out.