I think you're looking for a solution like **MaxiVista**, as already explained [**here**][1] (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.

![Small home-made graphic, just to see if I understood correctly :-)][2]


  [1]: http://superuser.com/questions/68431/software-based-dual-monitor-solution-via-lan/68432#68432
  [2]: http://img36.imageshack.us/img36/6272/captureut.png