I am still struggling to figure out the problem here. If there are two putlets and someone else uses one, that leaves one for OP. Perhaps a notional third passenger has an issue, but OP has not suggested this to be the case. Taken to an extreme, nobody should use the outlets ever.

Agreed that these things are best dealt with by a polite conversation. If that is worrisome, the cost of a dual adapter vs. a single adapter is so negligible as to be unimportant. In that case, the two outlets become three, four or whatever.