Do I need the master public keys of the cosigners in order to recover my wallet? The plan is to create a multisig with two hardware devices send some btc and then reset the devices and keep only the seed phrases.
It's not necessary since the master public key can be derived from the seed phrase or master private key,but in 'N-of-M' when 'N' is lower than 'M', you must back them up to guarantee that you can still restore the wallet in case one of the cosigner seed became inaccessible.
In terms of 2-of-2 MultiSig, don't bother backing up the master public key since as I've mentioned, Electrum will show you the derived xpub during wallet restoration.
Moreover, losing one seed phrase will lock you out of your funds.
You don't need to include the other cosigner's xpub with the seed since a single backup of "Cosigner1 seed" + "Cosigner2 xpub" will just restore the wallet but wont let you spend.
Just keep the seed phrases safe in this case.
In terms of 2-of-3 MultiSig (in case you want to include another cosigner), you can back them up as:
(don't replace xpub with xprv since one backup will have two cosigner secrets)
- "Cosigner1 seed" + "Cosigner2 xpub"
- "Cosigner2 seed" + "Cosigner3 xpub"
- "Cosigner3 seed" + "Cosigner1 xpub"
That way, even if you lose one backup, you can still restore the wallet since you still have access to the lost seed's xpub.
Since it only requires two signatures, having two seed phrases is enough to spend.
