To answer my own question, some lay research shows it seems it is technically possible for them not to merge if only a tiny portion of their apparent event hori

To answer my own question, some lay research shows it seems it is technically possible for them not to merge if only a tiny portion of their apparent event horizons merge and for only very briefly. But this is because of a distinction between the Apparent Horizon [1] (which is coordinate-dependent) and the true global event horizon. So they appear to briefly merge but no true global event horizon forms to encompass both. I think! [1] https://physics.stackexchange.com/questions/38721/what-is-the-apparent-horizon-of-a-black-hole#38736 https://physics.stackexchange.com/questions/38721/what-is-th...