Well nxserver permits session shadowing so you can connect to a single one a number of times, with or without the ability to meddle with it. Alternatively, I believe you can permit nx to connect to the existing X display, similar to x11vnc, using the nx-x11 module / library, but I'm not that aware of how to use that or if that's even correct advice.
Just seen this thread:
http://forums.fedoraforum.org/showthread.php?t=209586 which suggests it might be very simple to share the local desktop - other than their problems of course!