Heaviest pair of black holes ever seen weighs 28B times more than the sun

"Eventually, when the black holes are close enough together, their gravitational attraction should take over, and the black holes collide and merge just like the black holes that collided to create them did. The question is, could some supermassive black holes be so massive such a collision is stalled?"

Maybe such massive event horizons are "bouncy" and they need some kind of synchronization to merge?

By comparison, the mass of the black hole in the centre of our galaxy is around 4 million solar masses, and the mass of the black hole around M87 (the largest galaxy in our cosmological neighbourhood) is around 6 billion solar masses.

There's a 33 billion solar mass black hole in Abell 1201, but this system in Galaxy 0402+379 is the most massive binary pair. 24 light years apart.