How to stop a remote desktop session from ending

The situation you described would only happen if the two users are using the same username or you have hit the maximum concurrent users for the server (however you said there are only 2 people involved and the error you had would not happen till a 3rd person logged on and you don't have terminal services licensing set up).

If you are having the "same username" problem the two ways to fix it are to have the users use separate usernames to log in or to allow a single user to have multiple sessions open.

If you are having the "maximum concurrent users" problem you need to purchase a terminal services license and enable it on the server.

Go to Start > All Programs > Administrative Tools > Remote Desktop Services > Remote Desktop Session Host configuration. Double-click the setting "Restrict each user to a single session" and change it to no. Voila!

If the Terminal Server role service is installed, you can use mstsc /admin to start an administrative session that non-administrators cannot preempt. This switch has no effect if the Terminal Server role service is not installed.

For more information see :

Changes to remote administration in Windows Server 2008 section "How the /admin switch behaves"