TL;DR: It looks like only the final kill stroke receives xp.
This is a very disappointing and frustrating xp distribution method for coop play.
Some details.
There was three of us in the game. New characters.
We roamed the map together.
We were mostly what I would call "in close proximity".
One of the players was much more frisky. Meaning he was taking the kill on the majority of monsters.
At a certain point I stopped killing entirely.
Mr. Frisky was level 3 and I was level 2 (at 40% xp)
I kept very close to the action.
Some time later Mr. Frisky was level 4 and I was still level 2 (at 40%)
This is a very disappointing and frustrating xp distribution method for coop play.