This is not only a feature request for the Admin Area, but also for the Client Area.
Multiple other services supports adding a second (or even third) Yubikey as a "backup". I would really like to see this in WHMCS as well.
We're not only going to use it in case that a Yubikey gets lost - we just have multiple devices that we use to access WHMCS. It's a minor inconvenience to have to unplug the Yubikey from Device X and plug it into Device Y whenever we need access WHMCS from another device.
It would be so nice if we didn't have to use the WHMCS Backup Code in case we need to access WHMCS, but don't have that specific Yubikey nearby.