Google provides instructions for how to add a device to its two-step verification.
Add trusted computers and devices
Sign in on a computer or device you trust.
When you enter a verification code, select Don't ask again on this computer.
Unfortunately, these are inadequate. For example, on many devices I wish to add, I am already signed in. When I sign out and sign back in, there is no request for a verification code (so I can't select "Don't ask again on this computer"
Motivation
The reason for wanting to add more devices as trusted devices is because I recently updated my phone and Google's two-step verification will only allow me to carry out the second step of the verification on the old device. Thankfully, I hadn't yet wiped/sold it! Getting locked out of a Google account over something so poorly thought through would be disastrous.
Question
How do I add more devices as trusted devices for the purpose of two-step verification? (so that if one is not available, e.g. it's been sold, then I can do so on another device)
Best Answer
Google uses Google Authenticator for 2FA access. When you set up your 2FA there are 10 one-time use codes that you can use to regain access in the case of a lost device. Here is a link to find your codes (they recommend you print or save them somewhere safe)
Also as a backup, you can screenshot the QR code used to transfer your authenticator from one device to another to maintain access to your account. I used this method just this past year when my phone screen broke and stopped responding to touch. You would start the export and take a picture but then not complete the transfer. There are instructions for getting the QR code (for transfer) at this link and they also have instructions on setting up Google Authenticator on multiple devices for your 2FA codes.