Is this a way of earning double XP Points?
I think you can split the screen when playing Black Ops Multiplayer on-line, so that a mate can play on your system alongside you.
I've seen players on-line like this:-
So it appears that two people are playing online using the same console and login name..
If this is the case, do all the kills and XP points go to the one user... ie World_Peace in this example.