I've seen the army run with 12 accounts on one computer mowing down a fire spawn once, but I would never let it be used for strictly pvp Purposes. Back when I used to use it 24/7 I would always leave whatever I was doing if someone else was there trying to do the same thing, as I didn't feel right running multiple clients when they were only running 1.
Honestly, Running multiple clients on your computer is dangerous enough, if You get a client crash and don't actually close out UO, your CPU will hit 100% No matter your computer, or if multiple clients get disconnected and you don't have countermeasures in your scripts. Can fry your motherboard pretty quickly.