Here's how I did it:
I have a Matrox AGP slot video card that supports 2 monitors.
I then bough a PCI Matrox card that supports 2 monitors.
Generally - I've found that you should have the same brand of
video cards.

I have 4-monitor capability.
It's generally cheaper to have 2 video cards than a single card
that has 3 or 4 outputs, as they run a premium.
Even if you have a single AGP video card, you can get something for
under $100 at BestBuy or CompUSA that is a PCI slot dual output monitor.

When setting up the sim, I go out of "full screen mode", go to each
window and define the view (for example, rotating left or right to get
side views), then go back to full-screen mode.

There are some on line articles that describe it - I had to find
them through search engines.

Hope that helps.

Gary