Split-Task VDOM mode does not support virtual clustering. Split-Task VDOM mode supports all other FortiGate-6000 HA configurations described in FortiGate-6000 high availability, including standard FGCP HA, FGSP, standalone configuration synchronization, and VRRP.
To successfully form an FGCP HA cluster, both FortiGate-6000s must be operating in the same VDOM mode (Multi or Split-Task). You should change both FortiGate-6000s to the VDOM mode that you want them to operate in before configuring HA. To change the VDOM mode of an operating cluster, you need remove the backup FortiGate-6000 from the cluster, switch both FortiGate-6000s to the other VDOM mode and then re-form the cluster. This process will cause traffic interruptions.