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.
http://appleinsider.com/articles/11/03/31/mac_os_x_10_7_lion_to_introduce_multi_user_screen_sharing
http://knackforge.com/blog/vannia/os-x-mavericks-concurrent-remote-user-access-linux
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.
