You’d use AWS Organizations so each admin authenticates using their own credentials, gets short-term credentials to access the member account for the handful of operations needing root, and audit usage. It’s not only more secure, it’s also easier:
https://docs.aws.amazon.com/IAM/latest/UserGuide/id_root-ena...
Old school, you’d have a shared password in an encrypted team vault (possibly requiring x of y users to decrypt it) and two FIDO tokens locked in a safe. Again, this is rare and at a federal agency you have a physical security team with 24x7 staffing so you can say “in an emergency, one of the people on this list can get a key out of a safe in the CIO’s office”.