There has been many threads on setting up multiple installs of FS9. I myself asked that same question. I thought it would be a good idea to write up an easy to follow HOW TO on setting up multiple installs of FS9. It is really easy, though it does take some time. But it is time well spent as having multiple installs of FS9 allows you to categorize your aircraft into ERA/TYPE, allows you modify each install to better reflect the ERA/TYPE of flying you want to do in it.
I will also be writing up some HOW TOs on sharing mesh and scenery between installs, sharing sound packs between installs...ways that save HD space, ways that help keep things better organized and easier to find.
I will paste the text of the Multiple Installs of FS9 in the next post window.