How do I setup a second device to generate the two factor Auth key? I have it setup on my phone but would also like to have it on my backup phone or tablet.
I haven't used two factor authentication with TeamViewer, but it should be the same as with other services. You can use the same code to setup as many devices and authentication apps as you want
You can use Authy. I use Authy myself, but forgot the sync feature as i don't use it, but it can sync your two factor Authentications across multiple devices - desktops and mobiles.