Throw a, I can't go into too much detail (I might get fired!) but between two landers, the following can be cleared to land if the the preceding is beyond a certain point on the runway and assured of vacating, and between an arrival with a departure, likewise the departing needs to be beyond a certain point (still on ground) before lander crosses fence. There are other limiting aspects, such as weather, wind shear, turbulence, wet runways, daylight etc. In both cases the landing should be informed of the situation. Now if I get sacked, can I have a job at your place?