To produce a more powerful diesel switcher, it became commonplace to remove the cab from one example and semi-permanently couple it to another like switcher, (former) cab end to cab end, run control cables from the cabbed to the cabless one, enabling it to control both locomotives, and treat it like one larger locomotive. These permanently linked pairs became known, for whatever reason, as cow and calf sets.
This is different from standard MU working since the two locomotives are not intended to be detached from each other except possibly for maintenance, and they are generally not linked by normal couplings but by some kind of semi-permanent connection. The cow and calf set is treated as if it were one large, twin-engined locomotive that happens to be articulated in the middle.
Some cow and calf sets were built like that from the builder, and others were converted by the railroads themselves. They were generally used in large hump yards and other locations where a switcher, not a road locomotive, was wanted, for reasons of visibility, and power at low speeds instead of a high top speed. Some railroads, e.g. the Pennsylvania Railroad, did not buy such cow and calf sets but instead preferred to use road locomotives for hump pushers, but then the Pennsy had a tradition of doing so dating back to steam locomotive days.
Cow and calf sets were largely a North American trend, but British Railways built two-unit Class 13 sets for shunting at Tinsley yard in England.
They have pretty much died out these days, as indeed has the entire idea of the seperately designed switcher; instead, most railroads use older, retired road locomotives for their switching service.
One railroad, the C&O, purchased three-unit sets.