The custom_json in question look something like this:
First I would like to explain the purpose of these custom_json. The "3speak-publish" custom_json is triggered whenever a new video is published. This allows 3rd party apps to monitor the blockchain for new videos and create statistics. Another use case could be for anyone to build a 3rd party front end for 3Speak.
As you can see in the screenshot above there are two accounts listed next to "required_posting_auths". So why is that? It is a simple way to prevent anyone from faking these custom_json. Of course anyone can create a custom_json operation with the same id and json. But they won't have the posting authority for the threespeak account.
And this is why some custom_json posted by threespeak have more than one required posting authority.