not sure if coinbase has an upper limit as to how many you can register.
okay I have 2 yubi sticks but only did one.
I will look into doing the second stick right now.
A two second google surfing showed that you can link up to 5 YubiKey tokens to your account. The only thing I would add to the backup security key is a sealed CSV file + non-combustible paper. A couple of years ago, I also had a homemade security key on my keychain in one copy (but for a PC) and this gave me a lot of trouble since the password entry option was disabled,
(a little nostalgia).