It was a post about this on SN but can't find it right now. Search on SN is kind of crap showing results only for zaprank instead of recent/older.

Maybe @bitcoin_devs could give a better answer on this.
Check on their history too https://stacker.news/bitcoin_devs/posts

there's not two addresses in a multisig, there is one address, signed for with multiple keys

when that multisig output is spent those keys are revealed, but thats not the same as your extended pubkey... the derivation path is how basically you have infinite uncorrelated public keys from one true signing key