Complex sentence: With horses, one person could control both the motive power and the implement, but with a tractor, two were required, because the tractor required a driver and the implement, in many cases, required an operator.
Split into simpler sentences: With horses, one person could control both the motive power and the implement. But with the era's tractors, two were required, because the tractor required a driver and the implement, in many cases, required an operator.