Google only allows one "device" at a time, as far as I know. Adding a new device invalidates all old devices.

When you set up a new device, it gives you a new seed.

If you want two devices for Google 2FA, you need to either: - scan the qr code on both devices at once - or write down the seed for later use

Google won't let you retrieve the seed later (to prevent people from adding a second device with 2FA without your knowledge)