A Double Roundhouse Kick is also called a Wing Kick, Wings Kick, or narae chagi. It is somtimes called a Wing Kick because the two consecutive Roundhouse Kicks are reminiscent of flapping wings. (In Korean, narae means wing, so in Korean it would be called a Wing Kick.)
The idea is that one first performs a Roundhouse Kick with the back leg, then before that foot has returned to the ground you bring the front leg up to perform another Roundhouse Kick. From the standpoint of demonstrating good form, it's less important that either of these kicks be particularly high, and more important that the hips are completely turned over first for one kick then for the other. Failure to twist the hips so that they turn over could make this kick look like a Double Front Kick instead, which is why it's so important that the hips be turned over.
The intent is that the first kick is merely a feint. The second kick is the kick which should demonstrate power.