Given that, according to the news today, the two girls were related, it may not be suprising if some kind of formation flight (and I don't mean a formal close formation of any sort) was being done.
I've done a few formation flights for photo purposes and while we never got close - not anywhere remotely as close as the aviation magazine front cover pics are done at - it was obvious that serious precautions are necessary prior to acquiring visual contact, and again if contact is lost.
Recently I did a photo formation with a turboprop. It was set up that I got up there first and orbited a certain waypoint (an airway intersection; this was above a solid overcast) at 6700ft, and the other one was doing the same later at 6200ft but a few miles away. Despite having 3 in one plane and about 5 in the other, we found it extremely hard to spot each other even when we knew exactly where to look, and the background was uncluttered. Eventually we sorted it out, with me flying 10nm straight tracks on autopilot and him coming up within about 100m, and I got some great pics, but it's clear one cannot be casual about this sort of thing, where closing speeds are easily 300kt+.
It would have been so easy to try to look for each other at a similar altitude, and end up colliding. "See and avoid" is pretty damn useless when you really need it. It is great for spotting traffic which is nowhere near a collision course and that gives all the traditionalists a warm feeling inside.
Not suggesting this happened here but the two passengers being related would tend to suggest the possibility of some intentional proximity so they get pictures of each other - that would be pretty normal IMHO.