Very, at least for MSTS. I believe Trainz has support for multiple monitors but not in the manner you mean, not sure, it's support might also only be for the top end matrox cards too.
No, MSTS was only designed for a single monitor. However twin monitors are very useful for route builders etc. You can have the editor on one screen, and other programs such as MSTSPoker, text editors, Windows Explorer etc open on the other one.
You can even have Michael Vone's Guide to Route building open on screen two.
Mike in OZ - Author of TS-Tools & Route-Riter. http://www.agenetools.com
I'm not arguing (just explaining why I'm right).