I am running X on two displays. :0 is my primary display but I also have some programs running on :1. Is there any GUI application that will let me monitor display :1 in a window that sits on display :0?
(Using gnome, FWIW)
I know of 2 ways to do this when the system is remote, so given the 2nd X server is locally running on your system (:1) should really make no difference.
Method #1 - VNC
You could setup VNC on the 2nd X server and then run
Method #2 - XDMCP
If you want to remote display an entire desktop from one system to another system you'd typically use XDMCP. You can then use a tool such as Xephyr to connect to your 2nd X server on display
Here are several resources for setting up XDMCP and going further.