This feature was added in VuFind 8.0.
Typical usage of Shibboleth authentication in a multi ILS environment is as follows:
You configure Shibboleth as an alternative authentication method and library cards are connected to that account using ILS username and password (through ILS driver)
But in the case where you want to connect your VuFind instance to multiple Shibboleth identity providers and also enable connecting accounts through library cards, you have to set it explicitly.
The configuration option for this feature is in the [Catalog] section of config.ini and is called auth_based_library_cards. Just uncomment the example - set it to true.