I can only imagine having a computer able to handle that much at once lol
Well, I only have an i-5 3550 and a 460 GTX (but a fairly tightly tuned system) and was able to tri-box FFXI and play beta at the same time without anything dipping below 60 FPS (except for parts of Whitegate in FFXI and Ul'dah was around 50-55). Both games were running with most bells and whistles (FFXI instances were 1280 x 720, FFXIV 1920 x 1080) with the exception of shadows. Ironically the worst hits would happen in FFXI over FFXIV (shoddy porting + bard songs) but they'd be a few seconds at most.
It's definitely doable, but like he stated in the video he offloaded the other instances onto the second SLI/Crossfire card and dedicated the main instance to the first. FFXIV's engine seems so well designed now (that's not something you'd ever hear from 1.0...) that I could easily see such setups. All you'd have to do would be to just lower the resolution a bit on the master and cut down on some effects.