You need to use a different account or set the registry setting to allow multiple sessions per account.
If you end-goal is controlling both systems from a single mousekeyboard between the Mac and PC you’ll want to install [url=http://code.google.com/p/synergy-plus/]Synergy+[/url] instead.
Note: If you are only using Windows machines then I prefer [url=http://www.inputdirector.com/]Input Director[/url] over Synergy+
