Multiplayer Side-scroller RPG/Dungeon crawler like Darkest Dungeon

http://store.steampowered.com/tag/en/Side%20Scroller/#category[]=9&p=0&tab=TopSellers
.

You'll also want to check

http://store.steampowered.com/tag/en/Side%20Scroller/#category[]=20&p=0&tab=NewReleases
and
http://store.steampowered.com/tag/en/Side%20Scroller/#tag[]=1685&p=0&tab=NewReleases

To catch any games the first search may have missed.

Note that you'll need to check the first and third to see what kind of co-op they offer (some may only be local);
Also, since the third is user-tagged, there's always a chance it may be mislabeled, and more likely that they'll be local-only.
The third is the most comprehensive list, however.

Thx!

Hmm...
Mercenary Kings and Bro Force look also funny!