Each account will need a frontier account associated with it (email/password). The other point is that if you buy on steam you'll only be able to start that account from steam and not from the stand alone launcher. Steam will want to create it's own installation and create a duplicate.
You could probably start ED from the new steam installation for both accounts, one through steam and one through the stand alone launcher. Still this is probably a bit messy and you might prefer to end up with 2 installations of all the game files. It's not very hard to start 2 different accounts from the standalone Frontier launcher, maybe easier to buy it directly from Frontier, they'll most likely have a discount sometime over xmas.
But note that I'm not an expert on running Elite on steam, and others might have better advice.
