This question has come up in this forum several times over the years and I do not recall a single FTer who has confirmed BA has refunded the APD after cross referencing two separate PNRs. Not once.

There has been anecdotal evidence to suggest FTers have had limited success dealing with AA and UA who have agreed to refund APD on separate tickets, presumably as a result of a more laissez faire approach to their ADP obligations. Incidentally these carriers evidently take a more relaxed approach to paying corporation tax too but that’s fodder for a different thread in AAnother forum.

BA’s definition of a conjunction ticket can be found in BA’s General Conditions of Carriage - a ticket we have issued to you in conjunction with another ticket which together make up a single contract of carriage. That is pretty clear to me. Other airlines may have their own interpretation of a conjunction ticket and if they happen to be the ticket issuer and operating carrier of the first flight, then you may have more success with them, as it is the first uplifting carrier that is liable for APD.