Hier zwei Links die mir geholfen haben:

Man braucht "fast user switching" und eine termserv.dll mit der es klappt.

Bei Rechnern die in Domänen eingebunden sind, geht’s leider nicht so doll:

If the Windows XP computer is connected to a domain on local networks, Windows will set the value of the regkey "AllowMultipleTSSessions" to "0" every time the computer is restarted. Additionally the same user cannot login twice; you have to use different accounts. To ensure that multiple or unlimited Remote Desktop connection sessions is allowed in AD domain environment, the value data for "AllowMultipleTSSessions" has to be set to "1" on each system startup. For this you can setup a batch and start it via Autostart.

Geschrieben von Jan Niggemann in Computer und Technik am 19.01.2009