Fortigate HA

Hi, im trying to get my head around the topolgy/setup for a  hardware/site failover scenario. (diagram attached)

both sites have 1000D's as external and another as internal ( i know, i know :) ) running VDOM's.

site 1 and 2 have an active/passive external ISP link and 2 point to point links (layer 2) - terminated but not cabled anywhere yet.

i understand i can use the L2 link to create a HA heartbeat link between the 2 external firewalls, same for internal. so that i can have hardware redundancy, but am i right in thinking i would need the L2 link to expand the other connections such as ext/int DMZ and core etc... would i just cable the L2 into a switch and vlan the ports off?

thanks