The reason tacos have two tortillas is to keep them warmer and make them more filling.
Tacos al vapor are softer and are being kept warm under humid heat, so double tortillas makes the tacos sturdier as well.
Tacos also originated as a poor mans food so by having two tortillas to make the tacos made the tacos go further and made for a more filling food.
Tacos also get soggy and by serving the tacos with two tortillas instead of one it prevents the juices and grease from the meat from soaking through the first tortilla.
So most authentic taco places serving authentic tacos make the tacos using two tortillas.
Although some places do still serve tacos with one tortilla but a true Mexican place will serve the tacos with two tortillas instead of one.