Ok, i checked my working setup, and in the service menu, one cab should be master, one slave. Also in service menu, cars must be different colours - one red, one blue, for example. I don't think it will matter which is which, but for testing, to eliminate everything possible, I would leave the master cab's colour alone and change the slave.
Mame.ini from first cab:
#
# CORE COMM OPTIONS
#
comm_localhost 0.0.0.0
comm_localport 15112
comm_remotehost 192.168.1.100
comm_remoteport 15112
Mame.ini from second cab is identical, save IP change:
comm_remotehost 192.168.1.101