Valid HA topologies vary by whether you use FortiADC HA.
Figure 4 shows a network topology for FortiADC HA: the single FortiADC appliance has been replaced with two of them operating together as an
active-passive (high availability (HA) pair. If the active appliance fails, the standby appliance assumes the IP addresses and load balancing role of the failed appliance.
To carry heartbeat and synchronization traffic between the HA pair, the heartbeat interface on both HA appliances must be connected through crossover cables or through switches.