"Several twinned bows have been imaged, mostly during heavy showers, but currently there is no agreed explanation for them," writes atmospheric optics expert Les Cowley. "They might form from a mixture of water drops and ice spheres."
Indeed, Curtis notes, "hail can be seen falling right under the primary rainbow."
"A stronger possibility is that non-spherical raindrops produce one or both bows," Cowley adds. "Surface tension forces keep small raindrops fiercely spherical but as they fall large drops are flattened by air resistance or might even oscillate between flattened and elongated spheroids."