Well, it is possible... as long as your second screen is running something virtualized. So if you are working on the second screen using e.g. XP in Fusion, then you can have the screensaver on the second screen. If both are running Mac OS X, then no. Unless the other screen is with VNC or Remote Desktop connected to another Mac's Mac OS X, or you are running Mac OS X (Server) virtualized...
Are you concerned that an image might get 'burned in' to that second screen?
There's little likelihood of that happening on an LCD screen...
If you seldom use that second display, you could disconnect that screen. Plug it in when you need to use it...