I suppose that to meet your requirements you don't technically need to log in a second time, but just have screen real estate that does not exist on your primary monitor.

As such, you might want to check out MaxiVista. I'm not sure it'll do what you want, but it might. Also, KaVoom DesktopOne.

You'd still probably interrupt the primary monitor every once in a while, as I'm sure that new windows will at least occasionally pop up there instead of on the secondary, remote one.