I havent tried this, but I'm unsure.
Since you are splitting the XP half and half, say you get 100XP per mob you kill, so you each get 50XP, and you down the mob in 5 sec, so thats 600XP/Min or 36000XP/Hr, now im just making these numbers up.
But if you are solo, and getting 100XP per mob you kill, and I would guess it takes anywhere from 10-15 sec to down a mob, thats 480XP/Min or 28800XP/Hr, again just making these up.
So i would assume you would be better leveling in a group as you would die less often (very rarely in fact) and would spend less time resting.