I have 5 accounts in 1 b.net account.

Clicking the authenticator 5 times isn't as annoying as I first thought it was. Sure it takes a little longer... but I'm already used to it.

The chance that you can use 1 authenticator on 5 different b.net accounts and can log in with the same code is very low I reckon. I don't know if anyone has tried this already but it would be just as bad as allowing people to log in with 1 code on 1 b.net account, so I don't see any reason why blizzard would allow one but not the other.