You should be able to get connect to a shared calendar like this.
Instead of:
// load the default calendar
CalendarFolder calendar = CalendarFolder.Bind(Service, WellKnownFolderName.Calendar, new PropertySet());
use:
FolderId folder= new FolderId(WellKnownFolderName.Calendar, "another@mailbox.com");
CalendarFolder calendar = CalendarFolder.Bind(Service, folder);
See also:
https://stackoverflow.com/questions/41095971/how-do-i-access-a-shared-calendar-using-ews-managed-api