[AnotherUnixGeek]

“Every time two massive black holes collide,”

How common can this be ?

There's probably a reasonable chance of this happening whenever two galaxies with supermassive black holes at their centers merge.

[algore]

“There’s probably a reasonable chance of this happening”

yes, but how often ? Super Massive things will more likely tend to do slingshot orbital things IIRC

I would never expect to see this in my lifespan