If you just want to Peek (Read only Library), the content server (the CLI standalone) can be run on another library at the same time the GUI is running. You use a standard web browser for that kind of peeking.
The Library and related server, do not need to be on Your machine
Hubby can run a Calibre Content server on his Library, and you can view it with your browser (Assumes you share a network)
You can have Multiple Libraries on your computer and switch between them (library icon). They just can't be open in the GUI at the same time.
Some tools, can do operations across Libraries : Find Duplicates PI, Copy to Library