I'm being a bit technical. I don't know if you can really make the argument that it's not more secure but it doesn't make guessing the password any harder and you might as well do a 2 of 2 multisig. What this adds is a potential for geographic separation of two parts you need to recover. The problem is a passphrase is user supplied and people are bad at making passwords and you are far more likely to fuck it up or lose it then any password you choose because it can probably be guessed. If you want to split a key you can just make 24 words and split the 12 words geographically and get a "more secure" result than 12 words with a passphrase.