You create your first gateway as an EXL cluster gateway. Connect up the interfaces you want to connect to your network, there's presumably an internal interface (that could also be the management interface, magg1) and an external interface. SIC it to management, get a policy on there, make sure it all works.
Install the second gateway. Don't do the FTW, just have it freshly installed. Connect the Sync interface back to back with the first gateway. Connect the other interfaces to the same network segments as the first gateway. Each gateway in an EXL cluster is a semi-independent node on the network, requiring connectivity on the same interfaces to the same networks, much the same as a CXL cluster. In fact, cable it exactly the same as you would a CXL cluster. Then the second gateway will be available from the Cluster interface in the WebUI of the first gateway under Pending Gateways. You can add it to the cluster there.