I get a message that now I am controlling the secondary one, but I cannot see it on my primary monitor.
End of quote
Multiplicity is not a remote desktop application. It is an application that allows your mouse and keyboard to go from one PC's monitor to another PC's monitor just by moving your mouse across computers It's similar to a KVM switch, without the switch and with file and clipboard copying.
You can type on your single, primary keyboard and the key-strokes will go to whichever PC has focus. You will not see the secondary screen contents on the primary monitor.
Once you add the secondary computer on the primary grid representing it is in the physical world, moving your mouse across computers is natural. (http://sd.stardock.com/zubaz/images/2012-11-13_1438.png)