CC&R''s define a roof deck as Common Area. However it is given Exclusive Use status to one Owner. Is one or both Owners financially responsible?

To answer your question, a review of your CC&Rs would be required, and perhaps even the minutes in which the Board modified the Common Area you describe to Exclusive Use. It does seem odd at first glance. Financial responsibility for common areas is normally shared among all owners, not the "one or both" you mention.