There aren't any particular technical hurdles to a Touhou co-op mode that I can see. However, I'm not sure how it'd add anything to the average Touhou shoot-em-up (since Touhou is primarily focused on dodging, which is more of a single-person individual experience) nor am I sure how it'd NOT detract anything from the focus on dodging, either.
For example, a lot of Touhou at the higher difficulties is carefully gauging your own hit box so you can squeeze in between the bullets. In many cases on Lunatic difficulty, the difference between life and death boils down to a few pixels.
Now, in a two-player situation where both characters are near each other, can you imagine how HARD it'd be for the player to calculate where the pixels of these hit boxes lie when there's a second player character overlapping on top of him or her?
Going back to the "What does it actually add?" issue, the nature of Touhou is such that in a two-player mode, most players will be too focused on their own character's dodging and shooting to pay attention to what the other guy is doing. In this case, you almost might as well be playing alone instead, the one exception being noticing that your pal is about to die and using your own bomb to save her. While awesome if you manage to pull it off, for the most part that'd be... well, the only thing a two-player co-op would really add, I think.
Not that I wouldn't mind such a feature anyways, if only because it'd look pretty darn cool. It's just not particularly necessary. It's not like, say, New Super Mario Bros. Wii, where cramming four people onto the screen where they can do all sorts of wacky hijinks to each other such as throwing each other around or stealing each other's power-ups leads to ridiculous amounts of absolute HILARITY. When it comes to Touhou, the amount of ways you'd be able to interact with your partner are kinda minimal.
Again, there aren't any technical issues so it's still possible. And there are lots of games where two-player co-op ended up making no sense (like say, Battletoads) yet two-player ended up being awesome to play anyway (...at least for those who are masochistic. Anyone who's ever tried the ridiculously impossible craziness that was two-player Battletoads knows what I'm talking about. It's a game totally not designed for two-players and adding two-players to it makes a ridiculously difficult game even MORE ridiculously difficult, and yet many of my fondest co-op memories with my brother come from overcoming that difficulty hurdle in that game). ....but yea, it's just not particularly er... game-changing, I suppose is the way to put it? Necessary? Additive? Er, yea.
