The Thermaltake Mozart, as mentioned above, can hold two motherboards, but there are two problems with that.
1) The heat created will not be beneficial to either system - You are better off buying two regular cases (It carries a price tag of upwards of $250, so you can afford two nice cases).
2) The second motherboard is an M-ITX if I remember correctly, not a standard BTX or ATX. These motherboards are commonly made by VIA and have on board processors usually rated at 500MHz. They aren't much good for much other than file servers for the average user.
Personally, I would get two nice cases instead of a dual-system case. They are huge, weigh a lot, expensive, and can create heat issues. That is, unless you are putting one motherboard in and using the other half for something like a nice water cooling system... That's where it would come in handy (although, I would still rather one large single-board case...)