Under banking regulations and Georgia law is it permissible to have both a custodian and an authorized signer on a minor's account UTMA?

An UTMA account is governed by the Uniform Transfer to Minors Act as adopted by your state.
The Georgia version of UTMA starts at Code Section 44-5-110.
You should check with your bank's legal department or its outside Georgia counsel to determine the authority of a Georgia custodian to add a non-custodian as an authorized signer on a Georgia UTMA account.
Generally, under UTMA only one person may be appointed a custodian and that person does not have the authority or power to delegate their duties to a third party. You need to check the Georgia UTMA statute to see if Georgia law is different or the same.