I'm currently running bookshelf M22 and on-wall M22 at the same time (+ center and sub) because they each have their own sound characteristics. I have them mounted vertically with each other because it introduces other sound problems otherwise. I've never tried on opposite sides of the room, but I don't think I would like that with both on the same channel. I plan to move the on-walls to the back of the room later, then wall mount bookshelf M22s with bookshelf M2's vertically above them. I think that would give me more of a wall of sound that just the M22's themselves. But the on-wall M2's might be a better choice for that.
I don't know if this helps any, but I thought I'd add my .02 anyway.
