Making both parties do it or they die doesn't sound like much of a contract, and there would be no way to enforce it, the game can't simply kill you because a contract wasn't met, you would each have to have someone hunt down and kill the other, otherwise contracting someone to do something or you kill them is more of a threat.
Perhaps there could be a talent to that effect, after a certain time the character has penalties applied to them if they don't fulfill the contract they entered into, then again, nobody would do it. It's a cool idea, but sounds hard to apply to the game.