If you are running the Horizon View Client - login in to the client and then right click on the Pool or icon for the virtual machine. You should get a drop down with an option to change the display to "fullscreen" which is a single screen or "all monitors".
If you leave it on all montors but restrict the pool to a sinlge instance it can take control of all monitors and display a blank screen on ay additional you have which is not ideal.
