So what is you want to achieve? Having to copies of Elite?
You can only own a single copy of any game per steam account, in some cases, extra copies, can be given away to another steam account. So if you want to buy it again on steam, you will need a second steam account for that copy. You should be able to buy the second copy as a gift on your main account.
There are some rules regarding steam accounts, one of which are you are ONLY permitted to run a SINGLE game at the same time on the account, so having two computers, using same Steam account on both, you are only allowed to run a single game, so you can't play Elite on computer 1 and you kid/friend etc cannot play another steam game on computer 2... This limitation extends to the family sharing option Steam has, so I can share my Steam library with you, and when I am not playing a game, you can play any of the games in my library, and if I decides to play a game, your game session will be evicted after a notification, to allow you to save etc.
Elite is a bit special, since to play Elite, you need to have Frontier account, this is actually the account that holds the license for Elite, as you are required to login with that account to be able to play Elite.
So for Elite, and buying a second copy on steam is not that much trouble, as you still need a second steam account (secondary e-mail), but you also need a linked Frontier account, so use same e-mail here, and from here on, you play either CMDR from the same launcher, by simply changing what Frotnier account that is logged on in the launcher. So you do not need to switch Steam accounts etc.
If you want to run multiple copies of Elite at the same time on a single PC, then it gets a bit more complicated, but I think you should be able to download the launcher from Frontier, so you should have the option to either use Steam version or the Frontier version, there should be no difference, except that Steam will keep it copy updated so you do not have to pre-emptively launch the launcher to update the game.
