Family sharing... you will have a family album where you can share photos also, with 50GB storage, you can share your iCloud storage between the 2.. so both of you get 50 GB but only 1 pays...
[doublepost=1525952937][/doublepost]Otherwise, create a common Apple ID so both of you can use it.
Move one of the Photos libraries to /Users/Shared on the Mac. Point both user accounts Photos settings to that library. Now anytime one or the other person imports from their phone or iCloud account, pics wind up in same library. Might have to deal with some initial protection issues, but that should be solved relatively quickly after first attempt or two.
Only issue with this would be what to do with existing photos in other library. Maybe export the import to Shared library.
Or as mentioned, Family Sharing.
Or, believe you can sign into other iCloud account via Safari, download the pics to keep, import to master library.