I have tested iTunes in several dual-monitor configurations. The result is always the same: If you turn on Visuals in full-screen mode, the *main* monitor gets the effects, and the other gets black.
The only way to do what you want is to keep the Visuals inside the window (i.e. not Full-Screen), and move it to the screen you want.
BTW: If you run Visuals in full screen mode on a PowerBook, it uses the full 1024*768 pixels screen, while on a "normal" monitor it alway reduces the resolution. BUT: If you run it on a PowerBook with attached external monitor, but the PowerBook display is the main monitor, the resolution goes *still* down. Seems like a bug to me.
But, hey, the Visuals are cool enough to get yourself a Cube and a monitor, ONLY to watch it while you're working :-)
/sascha