I don't know if it will help, but… Have you tried paying for the Apple Suppport service and trying to get them to intigrate your separate ID accounts? It really does seem unconscionable that there is no way to do that. That's as bad as the cryptocurrency crap where you can NEVER retrieve your crypto if you loose one of the two keys. Or, if you had bitcoin on your computer and anything happened to it, it was forever lost; my son mined 5 bitcoin from back in the earliest days when it just starting; but that computer flatout died and the data was irretrievable (likely because of the mining) – so the bitcoins were lost. So very dumb!