Anyone with a graphics card made in the last few years should be able to easily handle two screens at optimum resolution with no slowdown. GPUs have been more than capable of doing this for some time now. Just make sure you set your refresh rates on both to be high (>60hz), as this can be the source of most people's eyestrain and headaches.
Where you will see slowdown is if you try to run 3D things across both screens at once.. but I reckon if you are doing that in the first place you will probably have a high end card anyway.

In terms of 2x 19" or 1x 22" .. that's really up to you.
I've owned a 22" Iiyama crt (big ass mofo) for 7-8 years now and it is still in mint shape. It is plenty of space to me.. and even though it hits crazy resolutions I only run it in 1600x1200.
At work I use two 19" lcds and it is essential for my workflow.
I have since added a 19" widescreen lcd to my home setup. Can I live without it ? Sure... but its really nice to now be able to have Ableton open on the big screen, and dedicate the lcd for showing the plugins for the selected track.
Really really nice... those of you who have used Ableton will know that it sometimes is a pain to try to have plugins open and see what's going on underneath.

So the choice is up to you.. but ultimately the more pixels the better. I'd say if you have the money, get two of either model.