With a POUG, both passengers on a single booking must be upgraded not just one (unless the booking is split, but splitting the booking - if it’s even possible to split a BA holidays booking - will probably see the POUG vanish). So this may not help.
It’s also not possible to book one passenger in ET and the other in CE on the same booking at the outset.
One possible option is an AUP on the day of the flight, although availability and price can’t be guaranteed. AFAIK this is possible for just one passenger on the booking.