I do believe that the altitude of detonation is a tactical decision, based on target selection.
A commander might attempt to destroy some types of hardened bunkers by penetrating the ground before detonating, or completely destroy a fleet of armored vehicles or a single specific building with a direct strike on the fleet itself, or upon the building's roof. Destroying population centers, or industrial zones is another matter, where an airburst maximizes above ground damage distributed across an area.
The blast wave doesn't precisely "double up" as much as it smashes downward, and plows outward, with a reflection of the sphere bouncing back upward. This effect does result in two waves, above a certain height, closer in, near ground zero, with harsher effects on tall buildings, if there are any. But the angle of the reflection grows more acute, as distance from the center of the sphere increases, and the equator of the blast sphere widens. So, once you get out past a few miles from ground zero, unless your in a building taller than 200 feet (66 meters), it's still really just going to be a single front that slams the structure you're occupying.
Furthermore, while airbursts do take advantage of the principle of reflected force, peculiarities of the target terrain also play a role, in terms of both ground texture (stone vs. soil) and topography (hills and valleys). A city center with lots of concrete and asphalt will be highly reflective, but rural targets surrounded by agricultural soil will be absorbent and inelastic, and produce a distorted, lower fidelity reflection.
Topography, meanwhile, will produce additional distortion and blurred shadows, behind and around corners, so targets, set back, behind the crests of hills, or behind many layers of tall buildings, might enjoy a lateral shadows from both flash heat and blast forces, but not ambient heat after the wave passes.
