Two logins at a same time in Server 2003. But with XP?

I know we can login two users at a same time into a single windows server 2003 installed system. Is that the limit or how many can be simultaneously?
Is it possible with windows XP? When i tried to login in remote system the current users session is locked. Is it possible to work remotely into a system which would be used by the current user at the same time on his position?