Tried that the other day, and it wasn't having any of it. Mind you, the other monitor was a telly, so it might have been down to the vastly differing resolutions.
BTW, I discovered that if you boot up in OS9 with something attached to yer S-ViDEO port and set up TV mirroring (to watch a DVD on your TV, for example), then shut down, unplug the S-VIDEO and restart in X, it carries on thinking that there are 2 monitors and moves things about accordingly. Rebooting into 9 without anything plugged into S-video automatically disables TV mirroring, and then you can boot back into X and everything's OK.
Am I the only person to discover this? Or am I just being stupid?