In fact, the common definitions of running require that both feet be off the ground at the same time...otherwise it is not running...

From a Wickipedia article:

In humans and other bipeds, walking is generally distinguished from running in that only one foot at a time leaves contact with the ground and there is a period of double-support. In contrast, running begins when both feet are off the ground with each step. (This distinction has the status of a formal requirement in competitive walking events, resulting in disqualification at the Olympic level.)

So, the rule about not returning the floor after "jumping" can't simply depend on both feet being off the floor at the same time. Otherwise, the layup would be illegal.