I have a solution that would be deemed massively politically incorrect and would have Americans up in arms.

We all know about carry-on sizing boxes: "If your bag fits completely within this box, you can carry it on".

Equip the ticketing area with human sizing boxes. If you can stand between these two walls (distance corresponding to flight class), you buy two seats.

If an airline implements this, I ask only that, if a passenger is found to be too wide to fly during check-in and either chooses not to fly or is denied boarding because it's sold out, that they be given the option of going on standby or getting a refund for both tickets. Likewise, if the plane takes off with any empty seats and the "passenger of size" is able to take advantage of an empty seat, they should be given a 50% refund.

If a certain air route is only being flown with Beech 1900s or any other aircraft with 1-1 seating, then the "passenger of size" would be simply out of luck.