Can you get an departure A/C wheels off the ground while an landing A/C just crossing the threshold of the runway? If you do so, it turns out that you must have issued "cleared to land" to the landing one when the departure one was just on the runway, as i know, you can't issue landing clearance until the runway is clear of any traffic, and you also can't issue landing clearance as late as 2 miles from threshold, so the key point is when to issue landing clearance so that the minimum separation between depture A/C and landing A/C can be met.