Prerequisites for Active/Active HA

The same model
—The firewalls in the pair must be of the same hardware model.
The same PAN-OS version
—The firewalls must be running the same PAN-OS version and must each be up-to-date on the application, URL, and threat databases.
The same multi virtual system capability
—Both firewalls must have
Multi Virtual System Capability
either enabled or not enabled. When enabled, each firewall requires its own multiple virtual systems licenses.
The same type of interfaces
—Dedicated HA links, or a combination of the management port and in-band ports that are set to interface type HA.
The HA interfaces must be configured with static IP addresses only, not IP addresses obtained from DHCP (except AWS can use DHCP addresses). Determine the IP address for the HA1 (control) connection between the HA peers. The HA1 IP address for the peers must be on the same subnet if they are directly connected or are connected to the same switch.
For firewalls without dedicated HA ports, you can use the management port for the control connection. Using the management port provides a direct communication link between the management planes on both firewalls. However, because the management ports will not be directly cabled between the peers, make sure that you have a route that connects these two interfaces across your network.
If you use Layer 3 as the transport method for the HA2 (data) connection, determine the IP address for the HA2 link. Use Layer 3 only if the HA2 connection must communicate over a routed network. The IP subnet for the HA2 links must not overlap with that of the HA1 links or with any other subnet assigned to the data ports on the firewall.
Each firewall needs a dedicated interface for the HA3 link. The PA-7000 Series, PA-5400 Series, PA-3400 Series, PA-3200, and PA-1400 Series firewalls use the HSCI port for HA3. The PA-5200 Series firewalls can use the HSCI port for HA3 or you can configure aggregate interfaces on the dataplane ports for HA3 for redundancy. On the remaining platforms, you can configure aggregate interfaces on dataplane ports as the HA3 link for redundancy.
The same set of licenses
—Licenses are unique to each firewall and cannot be shared between the firewalls. Therefore, you must license both firewalls identically. If both firewalls do not have an identical set of licenses, they cannot synchronize configuration information and maintain parity for a seamless failover.
If you have an existing firewall and you want to add a new firewall for HA purposes and the new firewall has an existing configuration, it is recommended that you Reset the Firewall to Factory Default Settings on the new firewall. This will ensure that the new firewall has a clean configuration. After HA is configured, you will then sync the configuration on the primary firewall to the newly introduced firewall with the clean config. You will also have to configure local IP addresses.