There are very few games that support 2 monitors and you are correct, flight sim pro is one of them.
You can set up dual monitors in two ways;
1. buy a graphics card that has dual monitor support, ie two connectors on one card.
2. Two seperate graphic cards, one AGP (main card) and one PCI (secondary card). I recommend a little research as to which ones are the best and support what Microsoft calls Multi-mon.
Check out
www.realtimesoft.com, they have a cool program that adds some cool features for dual monitors, ie task bar extended on the second monitor, two different wallpapers, etc. They also have a very helpful forum for multi-mon FAQs.
Hope this helps.