GTX 460 x 2 SLI with 3 Screen Surround and Accessory Display Possible?

Okay so I found the lovely tool to tell me how I can setup a 3 screen surround and get an Accessory Display its not clear if that's with 2 or 3 GPU?

Essentially I am getting a mATX board (so I will only have space for 2 GPU). I got a Dell 2560x1600 which is just great for throwing on any information documents etc onto and 3 1080p monitors for the surround.

From what I can tell it looks like I need a 3rd GPU to run the Dell 2560x1600 monitor as an Accessory Display. Is this correct?

Thanks in advance for answering!