It is not clear to me if you have 2 headsets signed into the same account, or each of you has your own account. If each of you is signed in with your own account, then yes, each of you must buy your own copy. The alternative is to sign on to both headsets with the account that owns the game as the main account, and on one of the headsets add the second account and enable app sharing. Then you could sign in to the secondary account on one and the main account on the other. You would then each have the app available to play.
Alternatively, you could just sign in both headsets with the one account because Elven Assassin is one of the few games that you can do multiplayer even with both headsets signed into the same account. To play multiplayer, what you do is launch a multiplayer game on one headset and then let the other person know what the two-digit code you chose was. Then the other person goes into the multiplayer area and looks for the username. It will show as your user name plus some numbers. The second person then connects and puts in the passcode and you should be able to play together.