Click on Start > Administrative Tools > Terminal Services > Terminal Services Configuration.

Right Click on "Restrict each user to a single session" in the "Edit settings" section and choose "Properties".

Uncheck the "Restrict each user to a single session" checkbox and Click OK.

Click OK for the window that opens.

You will need to log off and log back on for the changes to take effect.

You will now be able to connect to multiple Remote Desktop Sessions on the same user account.