For Cloudpages can SSL and SAP be shared across BUs?In Domain SSL Certificates we are seeing the Cloudpage domain only for 1 BU where as for image and click we can see its shared across BUs.
Yes, this is indeed possible. A Cloud Page Private Domain can indeed be utilized across multiple Business Units. And since the SSL is domain specific, this will "come along" automatically. If you have multiple Private Domains, it is not given, they will be automatically applied to the particular Business Unit.
You should reach out to Salesforce support, and ask them to activate the domain on the MID of the Business Unit in question.