It would help if you provided some bits of information:

• which addresses (and masks) are assigned to each NIC (ifconfig's output should suffice, if available on the two boxes, otherwise the correspondent ip subcommand)
• routing configuration (route -n if available, otherwise the correspondent ip subcommand)
• switch configuration
• the code you're using for your tests

Looking at your previous code, it seems that each NIC should have a different address living in a different network (I assume a default of 255.255.255.0 netmask when I see stuff in 192.168.x.y, but CIDR lets you override this so please confirm). If this applies, it should suffice to create a different socket bound to each particular address (using LocalAddr) and use the one corresponding to the NIC you want to send traffic through.