Hi,
Your Reth interfaces will span the cluster. In my cluster, port 0 is for management (fxp0), ports 1 and 2 control/data, port 3 trust (reth3), port 4 untrust/isp1 (reth4). In this example, the trust and untrust zone Reth interfaces will be used to route traffic. On a 240, the ge-0/0/3, ge-5/0/3, ge-0/0/4, and ge-5/0/4 will be used on each member for trust/untrust data. I hope this helps.
John