With Windows 7 the functionality to use multiple monitors is actually baked into the OS. If you notice the advanced settings on the MSTSC (Remote Desktop) executable, you will see the “Use all my monitors for the remote session” option. If you are remoting into a version of Windows 7 to take advantage of the dual monitor feature, the version also needs to be Enterprise or Ultimate.
This is also available via switch options for mstsc. Take a look at the screenshot below: