Consider a mechanical malfunction like an engine failure on the leading aircraft in climb (with high power setting and low airspeed). This will induce a bit of drift and a reduction in speed, which will reduce the already marginal separation between the two aircraft quite rapidly; quite possibly to a point where it becomes zero.
Of course, one could think of hooking these two aircrafts flight warning systems up via datalink (or so) so that the following aircraft can be called to automatically start an evasion manoeuver by the leader aircraft systems. With military aircraft that allow their small crew the silk way out in case of need, this may be practicable. But would You want to try this with unharnessed passengers or worse with cabin crew walking the aisle with their heavy trollies? I really do not.