Instead of multiple installs, I'd suggest the use of this free tool, Sim Starter:
http://aviation.pero-online.de/wordpress/?page_id=105You can configure 9 situations, sceneries, and simobjects that I think is your purpose on this multiple installation.
ANSWER TO YOUR QUESTION:
Yes it is possible with boxed editions. I don't know with Steam. Both boxed and Steam can work together.
I have done in the past with FS9 and FSX in order to have different types of planes in each installation.
But those FSX will have in common the folders containing FSX.cfg and scenery.cfg. You'll need registry editor.