I’ve had a couple of mad scrambles getting access back on my developer account. (I spend WAY too much effort tweaking my phone).
I would like to keep 2FA on my account, but I would like to have multiple devices as potential keys. (Actually, my wife’s so in a pinch, if my phone goes kerflooiee, I can still get into my account).
I cannot figure out how to put 2FA on my account for more than one device.
I assume this is your Firefox account?
One solution is using your 2FA recovery codes, I recommend printing them off and keeping them in a safe place. You can always reset them if you misplace them or think an untrusted source has had access to them.
Or, if you reset 2FA on your account, you can add the private key to both devices at the same time (just scan the QR code with both phones).
There are also services like Authy, which will encrypt and backup your codes in the cloud. You then only need to remember your encryption passphrase (or write it down somewhere safely).
I have a copy of my recovery codes in various safe places.
Your suggestion of both phones scanning the QR code is a good one, but the fact that one cannot do so separately (i.e. log in with the first phone, and then generate a QR code for the second phone separately) is IMHO a flaw that should be fixed.