Considering that the game just got released, is unfinished (macro system, scripts system, etc. is in the game but not fully functional), there is no support yet from the modding community...it actually surprising how well you can multibox it. There are a tons of different issues that make it not impossible but somewhat inconvenient...but it is doable if you want.

As far as running it on one computer...so far I managed to run 3 characters on one computer with decent FPS...I havent tried 4 but maybe I will once the guest passes work. What is more annoying is that I barely can run it on my older machines for various reasons...for reference my stsme is (QuadCore 9450 Yorkfield OC@3.0, 4Gigs RAM, 8800GTS OC, Windows XP)