Configure SSH keys for production and DR server

Here is the case, we have successfully configured SSH keys (and hence password-less SFTP connection) between ProdServer-A to ProdServer-B. It works, or so I thought still yesterday. Yesterday ProdServer-B failed over to DisasterServer-B. When it did, the SSH connection failed. In ProdServer-A we get an alert saying ProdServer-B has changed (known-hosts) and could be "man-in-middle" attack (which is as expected, but we know why this is happening). So, my question is, how can we add keys from two servers (same hostname, but only one active at a time) into known_hosts? Or even better, how can we successfully avoid SSH connection failures if a production server failed over to disaster server? Suggestions are welcome. Thanks!