On Dead Island, I'm fairly sure that the way the experience system works is that both players receive money, health, and experience. It shouldn't matter who gets the kill as long as someone does it. Or... maybe I'm mistaken. In thinking back to when I've played the game with a friend, I know they were specifically looking to get powerful items to sell, being on a much lower level than I was. Kills might have factored into that as well, though since I was so high up I had to help take down enemies along the way so we'd get through. It might be the case that the one doing the killing is getting the experience, but if memory serves I'd imagine you're both getting points for that.