From my ovserations, each card only drives the displays it is connected to. The type of connection to the display shouldn't matter, but as a last resort you could experiment with that, too. For two displays, though, the GT210 might actually be too slow (or have too little VRAM). I ended up replacing mu trusty G92 8800GTS with a GTX460 1GB because the 512MB on the GTS were actually a bottleneck with two 1080p monitors. Afterwards, though, some choppiness remained so I figured it was a software issue. Then I found this thread
. The solution by twizzlerite to add a couple of regstry keys and force Aero to run at hgh performance worked perfectly for me and I enjoy a multi-monitor setup smoother than a single-monitor one (sometimes in Windows 7, there are problems with Aero and multiple monitors).
In summation, here are my suggestions: First, try disabling only the window transparency for Aero, as that usually tells you if GPU poerformance is low. If the choppiness goes away, apply the registry trick described above and see if it helps. What it does is to force Aero to run in DX9 Emulation mode which, for some reason, is much smoother than native DX10. If that doesn't help then your problem might be insufficient VRAM.
EDIT: I assumed you are using Win7.
In XP/Vista, there shouldn't be such glitches if the cards are indeed fast enough. In other OSes I have no experience with such issues.