Not really a 100% RPG, but borderlands works pretty well co-op.

BGTutu worked for my brother and I for BG1 anyways.

Another vote for Neverwinter Nights. It's D&D but easy to get into and I know several couples who play it. And if you don't enjoy the original campaign, I bet you can find a custom module to suit your tastes.

What about the more action oriented games? Me and my partner had a lot of fun together with Dungeon Siege 3, Diablo 2 & 3 and Titan Quest.

And also, MMORPGs? I know you stated LAN, but they are very accessible and in most cases you can do a lot in two man teams, I would recommend WoW or LotRO. It is perfectly playable ignoring all other players.

If you might be interested in trying out a FPS, then Quake II works extremely well in co-op.
Finished it many times over with my brother (note: he's not an avid FPS player but enjoyed it very much).

My gf absolutely loves NWN, we've been through a lot of custom adventures. And the best part is that you can find adventures close to your liking, be it hack and slash, roleplay, heavy on conversation and so on.
If you choose to try the official content you might find the orignal story a bit bland though, better start off with shadows of undrentide