combine photos from two apple ids
How do I combine 40k pics from one Apple ID with 50k on another phone
iPhone 12 Pro Max, iOS 15
Do you have only an iPhone / iPad or do have also a MacBook or other computer available?
Here an older post, but therefore you would need a MacBook.
Want to merge two iCloud photo libraries - Apple Community
As soon as you have both Photos Libraries on your MacBook you could use the paid version of PowerPhotos and merge them.
https://www.fatcatsoftware.com/powerphotos/
Ralf