Norwegian- Extra seat for self after original booking
 

I am traveling with a companion, looking to book the seat between us for an overnight Norwegian 737Max flight. I didn't notice this was an option until reading about seat assignments after booking. I have already called Norwegian about adding an extra seat, but they simply quote me the price to buy two more seats (a new one for myself and the extra seat). Booking just the empty extra seat alone would be much cheaper now, but my concern would be that when checking in, the agents would not be willing to link the reservations & give the seat away to someone else.

Any thoughts/experiences?