Lifeswitch - (I have no idea, what im doing)

Okay, so basically, I want to make this cool Lifesteal twist. You trade your hearts with other players, to get more hearts. If someone kills you or you kill someone, you swap your amount of hearts with their amount of hearts. I would also want it to be where people on the same team can't trade or kill each other so they don't try to workaround the system.