If your FTD are in HA pair you just need to make the changes on the active FTD appliance. once you make the changes for example in your case adding the new sub-interface, once you define with ip addresses,name,security zone and description save your changes.
for adding the sub-interface standby ip address. go to high availability and look in at Monitor Interfaces (you will find your interface name) click on it and add the standby Ip address. it will also give your the option if you want to add this interface for monitoring for failure purpose. now save it and deploy the poliy from FMC. doing this FTD primary and secondary will have the ip address of primary sub-interface and secondary sub-interface address.
please do not forget to rate.
