Well I just mirrored my displays and it works... kind of.
I tried running PacMan through MAMEwah and the primary monitor runs fine while the secondary chugs along at an unplayable jittery pace. I didn't try tweaking much, but I ran both monitors at 800x600 to reduce strain and still no luck.
This was with regular command line mame, not advancemame. Perhaps it just needs more system power to work, but my system is no slouch. Athlon 3200+, 1gig PC3200 ram, Radeon 9800Pro.
Perhaps I'm just not setting something up correctly, but I'd classify this first attempt as a failure. You definitely don't want to play any game on that secondary display, thats for sure.