It's pretty straightforward, you can check it out in my profile JSON or nostr:npub17veg2gt7ja4dnjhs23jyttvpqlde3ajevlxuumxwghnqmgqwuv4q78cc5z's. It looks something like this:

Structure:

content = JSON string with all the fields (including just the first NIP-05 for compatibility)

tags = Array with all the NIP-05s as separate ["nip05", "..."] entries

Other clients only parse content.nip05, so they see one. The ones that support it read the tags, so they see both.