Split-Task VDOM mode does not support virtual clustering. Split-Task VDOM mode supports all other FortiGate-7000E HA configurations described in FortiGate-7000E high availability, including standard FGCP HA, FGSP, standalone configuration synchronization, and VRRP.
To successfully form an FGCP HA cluster, both FortiGate-7000Es must be operating in the same VDOM mode (Multi or Split-Task). You should change both FortiGate-7000Es 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-7000E from the cluster, switch both FortiGate-7000Es to the other VDOM mode and then re-form the cluster. This process will cause traffic interruptions.