I have been playing MP via Internet with a friend, and after we've played together and try to start another game, the guest's character won't show up in the staging area when they join and they eventually get disconnected.  They can click on "select a character" tab and see the character on the list, but cannot select it. 
However, if one person makes a new char then they can join and we can play together, but of course it is no fun having a lvl 0 with a lvl 50 partner.  I've seen a lot of people post about this problem, but no solutions.  We didn't dupe items or mod or any of that, but we did do some trading when we picked up loot.
We were both using version 1.11 and playing on the Ehb map.
Please email james24255@aol.com if you can offer any assistance.  Thank you.