AFAIK Calibre installs into the account that's current when its installed - I don't think it has an option to be installed for All User Accounts or Only Current User Account that one sees for some software.
You could
a) install the portable version
or
b) use
calibres environment variables to relocate the the configuration and temporary data to a location that is accessible to both accounts. You will also need to relocate the library(ies) from
Users/<whoever installed calibre>/Documents to a location that is accessible to both accounts.
I would create a top level folder named CalibreData (preferably on a drive other than the system drive) that contains three subfolders named Configuration, Temporary and Libraries. I would assign the relevant environment variables to the first two, and move my existing library folder(s) into the third.
I like to 'reserve' my install of calibre portable as a laboratory to try out any ideas or look at new features. Although there is no reason why one couldn't have calibre portable installed in multiple locations - e.g. one for everyday use and another to play with.
BR