^ All you need is to sign in with your split screener and join the other person's lobby. It's as simple as the 1v1 method, except this method is better, but requires another controller.
I have been boosting like this, but sometimes I dont get my xp when i go back to the lobby? U know why is that?
You probably went over the max xp you can get per game, now that there's a limit. Try finding out who's host and the last 10 seconds the host leaves, you still keep you xp if you went over the limit, but I'm not sure if the host will keep it too.