If you click "Can't scan the QR code?" during device setup, the secret key is displayed.
You can then store that secret key VERY securely (think: printed in a safe, GPG encrypted with a strong key, or whatever) and add it to new devices without having to get all devices together and re-key them all at once.
If your secret key is exposed you lose all benefits of 2factor auth, so do not do this unless you have an extremely secure way of storing the secret. Mine's stored, but it's GPG encrypted on an SD card I keep locked up for disaster recovery. It's not going anywhere.
You still have to go through and re-configure once, to add your 2nd device and record the key securely, but after that you can add more devices with only the key.
Personally I don't recommend this; you're better off generating emergency recovery 2factor codes using the accounts UI and using them if you lose your device, rather than adding a 2nd device and adding to your attack surface. I've only done it because for various non-technical reasons I cannot keep the same device with me everywhere, and one of them is reset regularly.