Have you tested this solution? Unless something has changed since the initial spec, each handshake includes a usage counter, which the relying party sees and is

Have you tested this solution? Unless something has changed since the initial spec, each handshake includes a usage counter, which the relying party sees and is supposed to remember. If the usage counter ever fails to increase, then that means something weird happened (like two keys acting as one), and the site can reject you. There are crude ways to deal with this issue, which are fine if you intend for the second to be used only in case of emergency.