I've got five B.Net accounts, with a single authenticator tied to each.
So it is one code, to log into all five at once.

Basically I initially log into my mule, for AH related stuff.
Which includes mailing gems and eternals to my transmute teams.
This toon then hearths to Dalaran.

Then I log in with each set of five toons, for the transmutes.
So, this is seven more uses of the authenticator.

Once the transmutes are done, I log the single toon in again.
To do the JC daily, and to receive the transmuted gems.
And to post them on the AH.

Then whatever team I'm working on, or playing gets logged in again.