I believe it has long been the case (pun semi-intended) that certain musicians buy an extra seat for their instrument. (cellists, for example).

Some of those instruments can be worth hundreds of thousands of dollars, so the musician doesn't want them either in a storage bin or in a seat in a different part of the plane. They would make sure their instrument is seated next to them.

If this is correct (and I'm sure someone here can confirm or refute it), then the same procedure will work for POS's who purchase 2 seats.