How Do I Merge Two Photo Libraries?
Fix the dates in both first, then merge into one date-based structure and let duplicates fall out.
Merging by hand fails because the two libraries name things differently. The only thing they genuinely share is when each photo was taken.
So fix the dates in both, then write both into the same year and month structure with the same naming rule. Photos from the same day land together, whatever they were called before.
Takeout JSON Metadata Fixer can run over one library and then the other into the same destination, recognising anything it has already placed. It runs on your own computer and is free for your first 500 photos.