Having 2 engines connected together takes up length on sidings. The length of the engine pair limits number of cars that can be moved at any one time. Another problem we ran across is that when we paired up two of our cheep engines, they overworked our lower priced transformers. In selecting engines to connect together, one must take great care to find 2 engines that run well together. When free running on the same track, they should not loose or gain much distance over the length of the track.
Matched speeds are important.
A faster engine will slip and buck, and can overheat or derail, and the faster engine will tend to drag or push the slower engine, wasting much pulling power. One advantage of a lashed pair of engines is the additional pulling power that is gained. The engines pull more than they could individually and long trains do look good, even if they are running around a circle of track. The condition of early roads prevented horse-drawn wagons from dragging much of a load. The wagons would bump on rocks and bog down in the mud.