I have 2 domains in the company on each tenancy of office 365.
Domain A has user x mailbox with shared calendar . people in domain A, can access the shared calendar of user x
Domain B has user y with y@domainB as primary in outlook user y has also got account in domain A as y@domainA as second account in the outlook
user y cannot access shared calendar of user x@domainA as the connect button is greyed out.