Depends where the crossing point is as if an aircraft is airborne before the crossing point then vortex has to be considered depending on the wake turbulence category of the first departing aircraft. If vortex wake separation is not applicable then there are no applicable separations. However depending on the direction of turn of each aircraft there may be reason for the tower controller to provide separation or the radar controller to use headings to separate. I.e 1st a/c departs on rw36 with a left turn and 2nd a/c departs rw27 with a right turn. In this case a controller would use headings or hold the second on the ground until the first passes A4.0 then release the second to only climb to A4.0. For landing a/c the first a/c must be past the intersection before the second is cleared to land or the first a/c has held short of the intersection. For parallel runways vortex has to be considered if the distance between the runways is less than 760m. In the USA they have different rules and can actually clear the second a/c for take off before the first a/c has passed the intersection but I think they have a lot of provisos e.g. Cavok, daylight, dry etc but not sure exactly what happens over the pond.