The problem with that video is they don't say if the Mac Pro is running the game in OS X or Windows or Linux. I've never heard of this kind of monitor spanning with accelerated 3D games working properly on a Mac before.
Besides, that's totally impractical and only serves as a way to show off. So like you said, don't plan on using it for competitive play unless you want to get mopped all over the floor.
I think it is also a safe bet that it is a single video card with dual video outs on it, because Macs lack the ability to use crossfire or sli which would allow the cards to run and communicate together. (As far as I know anyway.)
Also, UT2004 on the Mac, when my G5 used to have two monitors hooked up, the second monitor is totally inactive and unused. Depending on an INI file setting, the second monitor either goes black during gameplay, or displays the desktop only. So I don't know if the UT2004 Mac client is even capable of playing on two screens at once and really doubt it can.