Not sure if this will interest you, but I played with two methods to have two copies of FSX on the same system - I actually have 3 original copies of the sim and I can't explain why I have 3 because I don't know...lol
These are not perfect but they will work, you can set-up a dual boot with Win7 and XP (or Vista - that is what I had and it worked fine) then install FSX as per normal on the new clean 2nd OS, the neat thing is if you are really into poking around you can simply copy your original install to the new partition and overwrite the fresh install then you have two exact duplicates. You do have to make sure you get all the config files and saved flights from the My Docs/FSX folder but it can be done.
The other option (this is what I use now for testing) is set up an XP Virtual machine (if you have Win7 it is easy) then it is the same as the dual boot, but easier, just copy the original install from the Win 7 partition to the XP Virtual machine and you are good to go. The XP Virtual machine set-up works very well for me, the only drawback is the 4 gig memory limit, but that is an XP thing anyway.
If you have not tried Win7 "XP Mode" (which is the MS way of saying Virtual machine) give it a try, it is very handy, and you will still have access to all of your Win7 drives and folders, they show in My Computer as Mapped Network Shares.