Now I get it, you have to have TWO (2) yubikeys, one of which is a backup, in case one goes bad you have the other. It makes perfect sense. Google is really enforcing the required use of yubikey. My google account has 2FA (Enabled) sometimes even when I type the token generated by the authenticator application, google sends another code via sms to my cell phone to confirm my identity, that sucks, it's not always that happens, but when it comes to security we accept it. Now it has become 3FA and not 2FA.
You just pay attention to the cookies, I don't know how yubikey works because I never had one, so I can't give an opinion, but when you check the box trust this device, it will only ask for your password, ignoring the second MFA/2FA factor, I suppose you are accessing from the same device, that you marked as trusted. Wow $45 each? or both? if you had one or both in this price range nowadays I would buy it on the spot. Here it costs around $150 each. On the web you can find a more affordable price.