I'm sorry to make a new thread as there's already one basically devoted to this very subject, but I discovered the mentioned method on my own before seeing it here but I used an extra step for 5x the experience.
I've been up for 96 hours and writing this from memory, so instead of Player A and Player B im just going to describe it as I did it using a Barbarian (A) and a Demon Hunter(B). Obviously I'm doing this to level the DH.
1. Barb and DH enter game
2. Barb kills kulle. Noone touches the soulstone.
3. DH leaves, creates new game and turns in quest (1)
4. DH returns to original game.
5. Barb leaves game and makes a new game.
6. DH turns in quest now that the Barb is gone. (2)
7. DH joins Barb's new game.
8. Barb (or DH) grabs soulstone.
9. DH leaves game and turns in quest . (3)
10. DH rejoins barb.
11. Barb leaves game and makes a new game.
12. DH talks to adria and gets XP. (4)
13. DH joins barb in new game and talks to Adria for XP. (5)
Again sorry if this is a repost that someone else already discovered, but I didn't see it anywhere. As my DH is 60 figured I would release it. The extra step is definitely worth it as it's not the trek to the stone but rather just talking to Adria. Enjoy!