How to only list posts that are in category "a" AND category "b"?

Edit: changed title to be more precise

I have two different types of categories: artists, and media. Each of my posts has at least one category of each type.

My categories are for example:
Alice, Bob, Charlie, Movie, Text, Audio.

And posts would have categories like:
“alice, bob, audio” or “bob, movie, audio”.

Now for example I want to list all posts that deal with “Audio” created by “Bob”.

But then using category=”audio, bob” I get a list of all posts that have either “bob” OR “audio” as categories. What I want however is a list of posts where the categorie is “bob” AND “audio”.

How can I do this?

Thank you

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