I agree with the previous analysis about fill-rate no longer being the bottleneck, but whatever the bottleneck is nowadays should see a similar boost with two parallel vid cards over just one.
There isn't much of a bottleneck anymore. The top cards play the top games at 60 FPS, at 1600x1200, with anti-aliasing and everything. Granted, developers will put it to use before long, but everything's pretty rosy on the graphics side right now. The real hardware shortcomings games run into are back in the CPU, trying to do good AI and physics.
The big problem I have with this thing isn't the technology of the idea itself, it's the company. Alienware puts components together to make computers; they don't make components. It's kinda like if O'Reilly's (or whatever auto parts store) decided there was a market for something better than Ferrarris, so they're gonna make something to fill that niche. The market doesn't exist, but it doesn't matter, because even if it did you wouldn't be the one to take a crack at it.