Syncing my wife's iphone photos onto shared computer

It. is easier to share a library that is not your iCloud Photos Library or your system photos Library.

The system photos Library is special, because it is always in use while you are logged into you user account, even if you are not working with Photos. If you want to share the iCloud Photos Library on your shared computer, you cannot use fast user switching and stay both logged in. You have to log off, before the other user can log in and use the Shared Photos Library. Otherwise it should work as Tony described. It is essential to set the "Ignore Ownership on this volume" flag for your external volume with the library, or you would have continually to repair the permissions on the volume.

Photos is a single user program, and the library is meant to be private for each user. We are supposed to share selected, handpicked photos that might be interesting to others, but not a one joint library.

Hi - No, they haven't.

Pretty much the only way to do this that I know of, is to use two accounts on the mac (each with one of your apple ID's used) and to put the library onto an external drive (correctly formatted and directly connected) then log onto the relevant account to sync your phones pictures into the external library.

If you were to go this way you'd need to make sure the external drive were properly formatted (APFS or MacOS extended (journaled), directly connected (not any sort of network or synced drive) etc as described here:

https://support.apple.com/en-gb/HT201517