There is no law that I am aware of that mandates you require all owners of the deposit account to become obligated on the overdraft line of credit account. However, keep in mind the overdraft agreement covers all overdrafts to the account. If you do not have all owners of the deposit account obligated on the ODL then they (the ones not on the ODL) have no liability to pay the ODL if it goes bad.
We do the same as PoP. We require all owners of the deposit account to be covered by the ODL to sign the ODL agreement. We do not require authorized signers though. They have no legal obligation because they are not owners of the account and our ODL agreements covers all advances against the account regardless of the party of the deposit account that causes the overdraft.
The opinions expressed are mine and they are not to be taken as legal advice.