I currently use PowerDVD v7 on an old Athlon 1600mhz system with a Radeon 9600 All in wonder card. On the DVI output I have a 22" 16:9 Acer monitor. On the other (D-Sub 15 pin) T 19" 4:3 Monitor. I watch movies on the 22". PowerDVD suspends the screensaver, which I want it to do, but is there any way to at least blank out the non-movie monitor. The 22" looks great running movies, but the 19" showing desktop is annoying after awhile.
You could always try what I do... hit the power button. ;-)
Otherwise, I'm assuming you're not in "Clone Mode" with your dual monitor setup. Have you checked PowerDVD for a setting that doesn't disable both monitor's screen savers?