Theres an Ap called Mulitmon. It is a great little program for people w/ multi monitors. i have not used it for a while and I am not sure about Vista/7 support.
one feature of this ap will do exactly what you want, as well it will allow you to use 2 differant screensavers. It also has alot of other great features that will make you think how did I ever get along without them.