As a matter of fact, you can't. It's something that we have explored all the way back in 2011 with the first TFA we offered through Admin Tools itself and had the same discussion in 2012 when we contributed the TFA feature to Joomla! 3.2 itself.
Even if you change the QR code, the account would have to show as username@domain, e.g. email@example.com. This is a unique problem with the Google Authenticator application itself. If you use any other application which generates compatible Time-based One Time Passwords (TOTPs) such as Authy or even 1Password you can name the account anything you want. Google Authenticator doesn't let you because it's centered on the idea of generating TOTPs for your Google Accounts. Sorry :(
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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