Is there a way to run separate requests for O/D pairs (A-B and B-A) to analyze journeys in both directions? If so, would the combined request include any duplicate information?

Yes, set both A and B as Origins and Destinations without passthrough properties; both directions are returned with no duplicate records.

This is possible by setting A and B to be both Origins and Destinations without passthrough properties. You will receive both directions in your analysis with no duplicate information.