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.
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