Packets are directed towards the server NIC 0 MAC address using the client NIC 0 and expect to receive packets back on the NIC 1. There is no L3 support (ARP).
This module is "dual" because NIC 0 sends packets to NIC 1 and vice versa, whereas the basic :doc:`fastclick-play-single-mt <modules/fastclick-play-single-mt>` module is only generating on one interface.
The aformentioned module is much more advanced though.
Suggested usage example in your test (omit the prefixed "//" that prevent loop import here)
//%import@client fastclick-replay-dual traceIN=path/to.pcap traceOUT=path/to.pcap NIC_IN=0 NIC_OUT=1