Good find, but I'm not certain how to get them to "talk" either. Crossover can install Mono for Windows, but I'm pretty sure I already had Mono installed in my bottle for compatibility with a few other Steam games.
With some tinkering, it could probably be playable via a custom-patched version of Wine. Either way, there's hope for Crossover sometime down the line. Hopefully someone with a bit more tech know-how will chime in to see if there's any possibility of getting this running.
Edit: I decided to poke around WineHQ. It appears as if they've managed to get it running, but not without a bit of hassle. (Some/all sounds don't work, but singleplayer/multiplayer is functional).
The bug we're running into regarding Dungeon Defenders is discussed here: http://bugs.winehq.org/show_bug.cgi?id=28401
The .NET 3.0 installer worked in version 1.3.26, but broke after a patch. Crossover 10.1.2 is using Wine 1.3.9, where this bug is still present. It is somehow related to the Font Cache 3.0.0.0 that was added in a patch, and can be worked around by another patch or, supposedly, by deleting the Font Cache 3.0.0.0 service in WINE. I'm not sure whether this can be done in Crossover, since Codeweavers does their own magic.