The goal of passphrases is to produce a verifiably false passphrase + seed phrase combination. It is not JUST another seed word, it is a way to prevent the owner of the wallet to be protected in case of situations where the owner of the wallet is held hostage unless he reveals the true set of seed words

I've seen too many stories like this that end tragically. It's why I don't recommend passphrases: they result in a brittle 2-of-2 key architecture.