Why should we only use Non Floating Internal IP for config-sync operation? Can we use say Floating Internal IP or External IP for the same?
If both devices are HA peers then only the active device has that floating self-IP address so it makes no sense and won't show up in the drop-down list.
Use two crossover cables, create a trunk and use a single non-floating IP address. If you can't do that then use an internal non-floating self-IP. Make sure Port Lockdown is set to Default.