Well, have four authenticators currently, five if you count the phone.

At the moment, all five accounts are each on their own B.Net accounts, with a single authenticator getting all of them into game at once.

Once that is no longer an option...

Either have a separate B.Net / Authenticator per account, and have some kind of a system to switch from one to the next... Which gets you a six digit input per account, without delay between them.

Or move towards all of your Warcraft accounts on a single B.Net account, for ease of account management.