my guarda ALSO has a mnemonic phrase (12 words), whereas my partner does not. I assume this is because I initially created the wallet and my friend joined? Does that mean that mnemonic is the master key or something?
Usually, the mnemonic is your backup. If your partner doesn't have one, what happens if his hard drive (or phone) breaks down? Multisig is only safe if you're absolutely sure you won't lose any of the keys.