Does blackhole merging break their event horizon segregation?
We're also not physicians, not even Astrophysicians, but we're physicists. 

One of the more famous physicists, Leonard Susskind, discusses in his excellent youtube lecture series on general relativity (I believe it was episode 7, which I've linked) that during a black hole merger the Schwarzschild surface (not a sphere anymore) deforms and 'comes out to get you'.

Otherwise one would run into the problem that early research into black holes had, when they were still called Collapsars. This problem was that a Collapsar would eternally collapse due to the gravitational time dilation and never actually form a Schwarzschild radius that the static solutions indicate should exist. Then also, black holes could never merge.

But this problem has been solved with the realization that the Horizon deforms during a merger. The Horizons themselves then merge, and A and B come into causal contact.