I bought biometric locks for their individual doors. The cost was $60 for a two pack. It allows access to the individual teen, myself, and spouse. The sibling can't enter without an invitation.

The rule was, if the door is closed --- leave them alone. If they're in a common area, and they want to hangout one on one, you leave them alone unless/until invited.

If you argue then you lose out on the next sleepover/event/outing/hangout.

I would try not to have overlap. So if I knew that someone was coming over, I would try to set something up for the other kid --- either with my husband/myself, their friends, or a trip to the library/grocery store.