There's not a whole lot that DotEmu are shoveling out when it comes to emulator-wrapped ports of arcade games that I can find that also give you the ROM files to use them another emulator like the DRM-free stuff on the Humble Store. Besides the DRM-free versions of the Metal Slug games, the only co-op stuff I can find are the Shock Trooper games (top-down shoot 'em ups), Pulstar, Blazing Star, and Ironclad (horizontal shooters), King of the Monsters (a kaiju wrestling game), and Sengoku 3 (beat 'em up), though I haven't play those games but I might get my hands dirty to try them out. They also have 2-minute demo of most of the games for Firefox or Google Chome browsers if anyone wants to try them out. But speaking of emulator-wrapped ports, a few weeks ago, did Sega put a new emulator front-end for the Sega Genesis games they got on Steam, and what's cool is that they also give you the ROM file of the Genesis games you own with them on Steam. Most of the co-op stuff are beat 'em ups like Streets of Rage and Golden Axe though, plus a Gauntlet-styled game called Crackdown (not to be mistaken for the game on Xbox 360), and if you squint Sonic 2, and 3 & Knuckles.