Dariusburst might be the most expensive cab but that doesn't mean it was most expensive to develop.
yes, i was watching a video of it on youtube yesterday. I first thought that the thing was being rendered on a single machine(one gpu), then the output being split between two monitors. But you can see a slight difference in the rendering of the two monitors(lighting, shading very slightly off, even though the seams of the object match up). What i think is happening there is that each monitor has its own dedicated gpu to render to it. so thats gonna be a bitch to develop.
Pretty sure they had both SLI and Crossfire in the days when Darius Burst was developed, so developing for two GPUs with modern APIs is nothing amazing.
But yeah multi-screen output from one GPU is nothing new. I've got an ancient four-headed GPU around here somewhere. Pretty much every modern GPU supports this now.