I tried this and configured a cluster setup with two serials. The first config starts but ends up "Waiting All Secondaries to Auto-Link" When i connect the secondary firewall to the internet to do ZTP nothing happens. It keeps waiting for something to happen.
Anyone with experience with this option and who got it working? Anything special you did (like did you connect the heartbeat interface in advance or not?), or just connect two FortiGates to the internet and it went without issue?