When you run a back to back (b2b) test on one test server, in order to see messages on the PCAP trace, you must connect the NIC ports through a Layer 3 router and set the Next Hop Address to the IP address of the router. This makes Landslide send traffic to the router first, then to another interface. Otherwise the messages are sent over the back plane rather than physical ports.
Or: Use port and process reservation, put all of the interfaces on the same subnet and put the two test cases on different processes.
Or: Use port and process reservation, with the test cases on different processes it also works when the interfaces are on different subnets.