I had quite some fun playing Splinter Cell: Conviction in multiplayer Co-op.
Also Magicka is quite a good bet, thought not exactly an RPG, nether is SC I guess xD
errhh, co-op RPG's huh. Hmmmm, errr. There's this GTA san andreas mod i think that enables online play, never really played it much but I've heard about it.
Yeey Minecraft?
If you like puzzle games Trine 2 wasn't that bad, its a bit short though. Has Co-op.
Now that you mention it all RPG's I know are pretty much offline...
Skyrim, Fallout, oh right! I guess you can set Borderlands as an RPG/shooter, that one has quite some nice Co-op value. If you like that then probably you'll like Rage too.
anything else... errrhh.. The Cursed Crusade was for me pretty fun if you get over the rather clunkiness at times. Its a sword and broad kinda game.
Resident Evil 5 wasn't bad with Co-op either, againn not exactly an RPG by my standards but it might do.
Dead Island if you like zombie games, that one was neat too.
Gears of War has co-op. not an RPG, again xD
I'm just stating things you might like - I don't think the RPG's are much about online gameplay as much as developing a character, unless you're looking for MMORPG's like WoW and SWTOR.
For me my old WoW days are pretty much the most interesting form of co-op I had, mainly because I usually played strictly with a friend or 2. the Arena also gave you quite some co-op gameplay.