Use windowed mode and stretch it across- dual monitors usually is bad for gaming because the center of your display is right where the split in the monitors is. You probably ought to stick with your 19" for now. Cheers.
It really all depends on the bezel (or the black part (usually plastic) that surrounds the screen)..the smaller the bezel the better (obviously). As beltzy said you will most likely run into problems with this as your center is right where the two bezels meet. Also, since your monitors are different sizes it will most likely look pretty uneven and bad.
The good news is you can game on one screen and run internet windows, media players, msi afterburner, or any other programs in the other window. It's also nice for doing any other work/entertainment stuff when you're not gaming.
The targeting crosshairs is right in the middle and you can't see it very well , the best option is three monitors since that eliminates the problem. If you have older monitors then the bezels are going to be pretty thick and you won't have an enjoyable gaming experience. I have a 27" and a 25.5" and I just use the one screen and the two screens for everything else.