You really can't merge 2 photo libraries in apple photos. You can share photos or albums with people, but they don't dynamically update between the 2 accounts. But you know what does allow what you are looking for? Google photos. You can create a partner account where both parties can see all photos on both google accounts, or make dynamic albums that will update on both accounts. The best part is that its free and unlimited, as long as you don't mind a slight loss of quality in your photos. Google photos recompresses everything, but i hardly notice. BUt you get a free 15GB if you want to retain original quality, and the upgrade pricing is comparable to apples offerings. The ios app also just backs up in the background. There may be no app for a computer, but you can go to your google account in a web brower.
I use both, I use apple photos as my main library and backup. But i use google photos as like a secondary backup and an archive of edits. Yes google photos doesn't over ride your edits like apple, and will back up the edits as a separate photo. It also is a lot easier to share photos to android users. Just share an album, and they can add it to their google photos account.
Give it a try and see if you like it. Its free.
