2. Add both for each site you use it for

3. If using gpg keys you masterkey lives on a USB key, use subkeys which get transferred onto both yubikeys

4. Lock one the USB key and 2nd yubikey in a safe* with the password you never use

5. If you lose your day to day keys, unlock safe

*safe can be an actual safe, a "secure enough" place in your house, a bank safety deposit box, etc... You can also have multiple safes, one on site, one offsite.