So if anyone comes up to you please ask them how they are going to make your pvp game 100% provably fair, if they say something like
Server hash - client seed - server seed you know they are full of shit, because one of those players can still be a "house" in disguise
Not true.
You simply need to factor in an abstraction layer that can be used to calculate the outcome of both client seeds. You would then use that outcome in a bisect.