I think you're looking for a solution like MaxiVista, as already explained here (and in tons of other discussions on SU). Synergy is something else, where each computer keeps its screen, only the mouse and keyboard are redirected.
However, I think it's a bit strange to extend the desktop on the screen of the other computer, and do a RDP connection to access it? But it should work, as the RDP connection will access another session and the Maxivista extended screen will use the interactive session.
