I've always understood that the terms Master and Slave referred to Radio - controlled diesels. Back in The Day, Southern Rwy & later Espee bought &/or modified certain diesels to perform either Master ( 1st unit in lead consist ) or Remote ( which could be any unit in 2nd set of engines ). The early Radio Remote Control apparatus was quite large, so that separate engines had to be used for each application. Santa Fe & BN used F - Unit boosters ( " B " - Units " ) with the prime mover removed, for the remote radio installation. Espee bought some SD-40-T-2 units set up as Master only, and others ( in a different number series ) for Remote equipment. These units were labeled MASTER or REMOTE on the cab sides.
I understand today's radio control equipment is small enough to perform either function, as long as the Lead unit & Remote unit are confirmed to be in contact with each other before departure.