You cant have two seperate users on one machine...but u can have 3 displays with one keyboard and mouse .
OS X is running on top of Unix....... Two users is certainly possible. That specific set up may have problems.
Multiple GUI users at the same time has been around for a while.
the local user over the built in video outputs (since all sharing local GPU frame buffer), but may not have to necessrily attach the Mac Pro to the TV directly. A couple of scenarios.
a. login in remote with a chromecast plugged into TV. From browser/whatever on login in session "throw" video to the Chromecast.
b. Similar if TV (or TV + Roku/etc ) box can pick up files severed out of the Mac Pro.
Not fulling up to speed about how the XMBC remote clients/remote controls work, but there might be a way to manipulate without having to touch the keyboard/mouse attached to the Mac Pro. ( e.g., the XMBC program is dedicated to the HDMI video output in it own "space" and remote control does all the 'clicking'/'selecting'. ) [ no IR receiver on Mac Pro makes the classic physical remote kind of hard, but there are virtual remotes. ]
There is no software that can make your computer read two seperate mouse and keyboard
On OS X. On some OS there are "boxes"/"add ons" can buy that can do this. I don't think those are particularly necessary or useful in the home audio video PC application space though.