Sometimes (not all the time) if you say the person's name first, it helps with unwanted replies.

I absolutely can't focus on one person if two are talking about roughly the same thing at the same time. The only way for me to take in any information is unfortunately to apologize to both of them and explain that while I'm glad they both know how to handle the situation I'm asking about, I can't handle the information overload of both of them explaining it at the same time.

At which point I have to re-ask the same person I asked the first time to explain it.

why are (some) extroverts like this?

I sometimes do this too even though I'm very introverted. I do this because I want to feel useful with the experience I gained and it just feels like a waste sitting on some knowledge and not being able to do something with it.

It's a really cool thing if you can help someone. And some people have such a need for this, either they completely forget they were very explicitly not asked, and some will ignore it, just in the hopes they get to contribute.

Funnily enough I get to see both sides, because I also sometimes get an answer from multiple people, so I've learned how to handle it to some degree.

The best thing to do is not to tell them to shut up, but to acknowledge it and then explicitly say "also wanna hear from [experienced] person as well on this though."

They (probably) aren't trying to be rude, they are just happy to have an answer they can give quickly and appear helpful. They are oblivious to existing conversations or the communication processing talents of others.