By design Remote Desktop on a workstation will only allow one user at a time to use the desktop. See this KB article for an explanation.
http://support.microsoft.com/kb/280828
If your at home and want to login to the PC at the remote site and a user is logged in they will be disconnected (NOT logged off) meaning their session is still active in background and they will not lose any data, etc.
If you need to do a help desk type function, ie. allow the remote user to see what our doing on their desktop while you help them, you might look at Remote Assistance (built-in to the OS) or Virtual Network Computing (VNC) or TeamViewer. Personally I recommend and use TeamViewer for this type of function, ie. I support a couple of my sister-in-laws remotely.
MS-MVP Windows Desktop Experience, "When all else fails, read the instructions"