((In the interview, it was never actually *said* they work together. Joystiq just had that impression. Also, the invasion is led by Varian, so I think it's a stretch, even for Warcraft, that Horde forces would follow an Alliance King. They would wanna deal with their own dirty laundry.
And as far as warmongering, Garrosh is actually much worse than Varian, who has every reason to be as aggressive as Garrosh, but hasn't been. Other than some land in the Barrens, which we're gonna lose, Garrosh has been playing offense, Varian defense.))
(( Western Plaguelands, Swamp of Sorrows, the Barrens.
Just because they haven't been particularly /winning/ doesn't mean they've been playing defensively.
That said, the whole corrupted bit, if the Horde /doesn't/ turn against him is pretty ridiculous. This isn't the old Horde, the Horde isn't ~eeeeviiilll~, nor villains. If Garrosh turns evil, we should likely be there to kill him too. ))