Unfortunately, there's nothing you're "doing" or "not doing" wrong. It's the nature of the beast when running NVIDIA cards with dual monitor outputs.
You're using an older NVIDIA graphics card... roughly 2 generations behind current, which is one issue, mainly because (I'm assuming) you're running the two monitors out of the same card (both DVI ports) and using NView to manage multiple monitors.

That's how your framerate is dropping so drastically. Most people (including myself) have noticed a drop of around 50% of the normal (ie single account running full screen) frame rate. There is an issue with using the "span desktop" function in NView. Couple that with the drop in framerate you'll get when switching from full screen to windowed (that's the only way I've found to get the games showing up correctly, although I have a 22" widescreen and a 20" normal screen that I use).

I don't have any answers for you, as I just built up an old AMD 3800+ system for my clones to run around on... but I was having the same frame rate issues as you were, using an AMD 5000+ BE overclocked to 3.1GHz, 4 GB of DDR3 RAM, and an 8800GTX video card. I could run all day maxed res/graphics and never hiccup. I added in the 20" monitor and it was slower than my integrated graphics card on my work laptop. Go figure.

Sorry I couldn't be of more help, but I figured this way you won't go nuts trying to fix something that isn't broken or set up incorrectly.