I would have thought it was a very simple matter to work out exactly how close they were, seing as the aircraft types and therefore their dimensions are obvious.
The lower one appears to be in a slight turn which would make the arithmatic a little more complex, but the silhouette could be studied to get the bank angle to use as an adjustment, blah blah etc etc, I'm even boring myself.
It's just simple trignometry. If we can send a probe all the way to Mars, how hard can this be?
