This is from 5 yrs ago, but should still work

Follow this thread, you might wanna go bottom up
http://www.golod.com/enabling-multiple-remote-desktop-sessions-in-windows-xp-professional-and-media-center-edition-2005/

The thread contains links to download the hacked DLL files with
unlimited connection. And someone made an installer
http://www.kood.org/terminal-server-patch/  (Working patch)