Original: Multiple working is a term used on the UK rail network to describe the practice of having more than one diesel or electric locomotive hauling a train under the control of one driver.
Simplified: The term can also apply when diesel or electric multiple unit trains are coupled together and also under the control of one driver. 'In tandem' is the term usually used in the UK when more than one diesel or electric locomotive are hauling a single train and under the control of a driver on each locomotive.