Personally I think Alienware's patent on this is unenforcible, and it should be. There's too much prior art that has basically done this before. A good example would be the Voodoo 2.
If Alienware tries to use their patents to stop another company from developing their own 2-card system, they'll lose in court (if they sue).
There are two situations with Alienware's patent, as I see it. In situation one, they've applied for the patents as a delaying tactic; they know the patents will be invalidated, but hope that the patents will provide them with a bit more time as the only company with such a solution. If this is the situation, I fully support Alienware. This gives them a momentary advantage in the market.
The other situation is that Alienware is serious about the patents on dual-videocards. In this situation, Alienware is being extremely unethical. They're patenting an obvious technique that has already been done in the past (prior art), and attempting to monopolize it. There have been systems with dual-videocards before, not to mention the fact that AGP and PCI-Express were both designed with support for dual-videocards (Yes, later revisions of AGP had support for a second AGP slot. Nobody ever made a motherboard with it). Can Alienware patent something that they didn't invent, and didn't design (The PCI-Express spec already handles the hardware specifics, IIRC)? If this IS the case, Alienware should be ashamed of themselves. They're holding back technological innovation. Could you imaging of Intel had tried to patent 0.13u CPUs in general? And nobody else could make any CPU with a 0.13u process? Of course not, it'd be rediculous.
Personally, I hope very much that the first case is true, that this is only a delaying tactic for Alienware. Because if they actually try to enforce this patent, they'll be... well, evil.
That said, had they gone the external route, I'd have no trouble with them patenting that, I don't think anybody has ever made a viable external videocard enclosure for use with multiple videocards.
EDIT: I should mention that, even if/though this system supports the use of, for example, one ATI videocard and one nVidia videocards, you shouldn't do that. If you do, you'll get lower performance (as one card will invariably be slower than the other, pulling the other down), as well as the fact that ATI and nVidia videocards can render things very VERY differently at times, and you'd see an obvious line through the center of the screen where the different images met. And can you imagine having both ATI and nVidia drivers installed at the same time?
You could probably get away with two cards of the same brand; IE, Radeon 9800 XT in one slot, and a Radeon 9700 Pro in another. They'd use the same drivers and be similar enough in filtering techniques with the proper settings to match up.