In Windows Server 2008 and Windows Server 2008 R2, multiple remote desktop sessions is not configured by default.
Multiple RDP Sessions
Here is a quick way to enable it.
Launch your start menu (Win Key), and type gpedit.msc.
Under Computer Configuration browse down to Administrative Templates > Windows Components.
Then browse to Remote Desktop Services > Remote Desktop Session Host > Connections
Right click on “Limit number of connections” and select Edit. Select Disabled.
Right click on “Restrict Remote Desktop Services users to a single Remote Desktop Services session” and select Edit. Select Disabled.
You should now be able to connect with at least 2 simultaneous users through RDP to your server.
For more information you can also read Microsoft’s article: “Configure the Number of Simultaneous Remote Connections Allowed for a Connection.”