Many times, you’ll find it necessary to have multiple connections to a Remote Desktop server, and by default, Windows Server 2008, Windows Server 2003, Window Vista, Windows XP, etc. limit the number of Remote Desktop Connections to 1 user.
On Windows Server 2008, the Restrict Terminal Services users to a single remote session Group Policy setting determines whether you can connect to your existing physical console session. When this option is enabled, when logging on by default, you will be restricted to a single remote session.
This setting is available in the Computer Configuration\Administrative Templates\Windows Components\Terminal Services\Terminal Server\Connections node of the Local Group Policy Editor. You can also configure this setting in Terminal Services Configuration. The Restrict each user to a single session setting appears in Edit settings in the General section.
If you would like to have multiple simultaneous sessions for the same user, please uncheck this setting or set this group policy as Disabled.
For more information regarding Changes to remote administration in Windows Server 2008, please refer to the following KB article:
Changes to remote administration in Windows Server 2008