Currently, there only is only space for 1 Yubikey assigned to an Administrator account.
This makes it impossible to follow Yubico’s recommendation of always having 1 primary key and 1 backup key. It also inhibits a scenario where multiple Yubikeys are in use depending on the location (eg. 1 Yubikey at home for desktop usage, 1 Yubikey attached to car keys for mobile usage).
This is a feature request to add support for an additional Yubikey to be used on the same account.