Should tacos always be doubled — two tortillas instead of one?

Both structural and traditional. A single corn tortilla can tear under a wet or heavy filling — corn tortillas are thinner and more fragile than flour. The double tortilla distributes the weight across two layers and the moisture from the filling has to penetrate two tortillas before it reaches your hand. The second tortilla also absorbs flavour from the first bite and becomes its own small thing. Single tortilla tacos are legitimate in specific styles (certain regional tacos use one thick tortilla) but for a standard street taco, double is correct.

The best tacos I've eaten in Mexico were always double. The second tortilla is also a signal — a taqueria that doubles their tortillas is one that cares enough about structural integrity to spend the extra tortilla on it. It is a minor quality indicator.