Just a note: I had, for quite a while, FSX:SE on two computers at the same time. Same Steam account, namely mine. The only thing was, only one of the two could be online at once. Not a real big problem for me. Essentially, I did it to show my lovely wife the differences between FSX:SE and FS9. After a few months of flying in SE she decided she liked it better, so we purchased an account for her. Essentially, she wanted to fly with the two of us together in MP.
Point is, you can have the same FSX:SE loaded on more than one computer at a time, as long as you only want one sim online at any given time. Not the same computer, as far as I know, but I may be wrong on that. You can still run both installs at the same time, just not online or in MP with one another. Mainly, it started out as a vanilla back-up install for me, and then she decided she wanted it as well, so we got her set up with her own account.
My vanilla backup install is still on her PC right at the moment. I know, with SE it's not really necessary, since you can re-load, or repair, an install from Steam easily, just a habit I developed with FS9, having a vanilla backup hidden away someplace.
Steam may have changed something preventing this set-up now, but it worked for me.
Pat☺