Originally Posted by MSPeconomist

My guess is that the third seat needs to be booked with a name something like XTRASEAT. That's the procedure with other airlines and often you must call to do this.

Originally Posted by Often1

Absolutely. Call F9. Don't try to do this online and run the risk of one or both tickets being cancelled as duplicate bookings.
You will also receive specific instructions as to how the situation is handled. It is much more complex than simply having the GA scan the extra BP as that throws the pax count off and the aircraft can't push until the count is reconciled.