A very interesting question.
Tail lights, as the name suggests, were first made from carbide in the 16th century, and were attached to squirrel's tails, to enable the squirrel hunters to locate and hunt them at night in the forests.
This in turn led to confusion at night with the hunters who rode on horseback, colliding with each other. Thus the carbide light was attached to the horses tails. This worked for a short while but then several accidents occurred when the horses were mistaken for squirrels at night and several hunters were accidently shot.
This was overcome by using horse and carriages. The carbide lamps were then put on the back of the carriages as opposed to the horses tails. Each carriage required two lights because of it's width. This solved the problem and the rest is history.
The tail-lights became mandatory in 1868, when the population of squirrels was almost wiped out and there was no need for the horse and carriages to go into the forests hunting anymore. This caused congestion on the once empty roads, especially at night, so it became law for dual tail-lights. Hope this helps.