Right now I've got one 1080p Samsung monitor (= set as main display), and one older Dell 1280x1024 monitor. If I plug in my Rift, it's recognized as a third monitor, however it isn't actually 'activated'. If I try to clone either my Samsung or Dell screen to it, even after changing the res to 1280x800, it just says it "can't save the screen settings" (translated, since my Windows is in Dutch).
In order for the Rift to actually work I unplug my 1080p monitor, then I can clone my Dell display to my Rift display and I'm ready to rock. I know the Unity guide mentions you have to clone your main display, but it seems you can't have a secondary display plugged in at the same time. Known issue?