Thx muchly for the insight. Lots to think about.
The 2nd installation (wife's) would be only for viewing the library (no mods or book additions) so read only would be okay. How is this done in windows? Mark the folder read-only? Actually, it would be preferential that the wife CAN'T change anything in the library.
I would like calibre to have the same plugins and columns / settings etc though on the 2nd machine.
I have some tinkering to do I see. Thx again.