That's interesting that they both launched off 09L. As said above the PIA was slightly south of the normal and the THY slightly north. Perhaps it was that the THY was going a bit faster and was 'catching up' with the PIA 777, because from my viewpoint the time gap from one to the other passing was very short. On reflection, no more than 30secs, perhaps less. Visually, it was very out of the ordinary. A friend says he remembers 2 VC-137's coming out of LHR almost simultaneously back in the early 1970's during a presidential visit.