At this time, each request can only specify a single audience so the scopes would also be the ones associated with the specified audience.
Do you plan to add that kind of functionality in the nearest future ? I mean - to be able to send request for access token that handles multiple audiences. I am aware of the other ways of handling multiple APIs but none of them seems to fit for my use case. I have multiple systems with different APIs that cannot have the same audience.
I would really suggest directly getting in touch with our product team sending them a case detail-oriented description what you want to achieve what are the roadblocks etc, using our feedback site:
to potentially submit a feature request. Thank you!