Gawd man this would suck major, major ass. They were meant to be seen together.
Though, I imagine the first release would be sepearte....then a few months later probably do what New Line did with LOTR and have the full Grindhouse (with the deleted footage back in) on like 2 discs with another disc or two of special features.
Or do a Sin City with theaterical version on one disc, then the movies seperate with the deleted footage back in on another plus a disc or two of special features.
Either one of those I wouldn't mind.
But, I agree that a single disc of 1 film and a disc of the other will probably hit...but a combined set will probably come out a few months later.