I found a few articles explaining how to enable Concurrent Sessions for Windows 7, I have not tried nor verified them as I use Linux which is by default able to handle many concurrent sessions through ssh. Hope they help.
http://www.blogsdna.com/4560/how-to-enable-concurrent-sessions-for-remote-desktop-in-windows-7-rtm.htmhttp://www.hack7mc.com/2009/02/enable-concurrent-user-sessions-in.html