Use one library on several computers
Open one photo library on several computers — kept on a NAS or another shared folder — and the albums, tags, stars and edits made on each reach the others.
Each computer is a replica: it keeps its own catalogue and thumbnails, and they exchange what people do.
Setting one up
On the second computer, create a library and give it the same photo folder. The form recognises the folder as an existing library and joins it. The originals folder must be reachable from both computers, though not necessarily under the same drive letter.
What is exchanged
Albums, tags, stars and edits made on any computer reach the others. No computer is in charge: each can make changes. They are exchanged directly over the LAN when the other computer is on, and left in the shared folder for one that is switched off, to pick up later.
Each computer makes its own thumbnails, but takes them from another computer that has already made them where it can, rather than making every one again. A new replica can also take the catalogue from another, instead of reading every file itself.
Keeping an eye on it
Settings ▸ Sync ▸ Replicas shows how the exchange is going and which other computers the folder knows. If this computer stops exchanging, a notice says so, with Show me.
A computer without FFmpeg will lack the library's films; the Replicas pane says so.
To exchange changes even while Portman2 is closed, install the background host in LAN server settings.