When black holes meet there are two things to consider when contemplating the merger.
- Adding mass to give just the required angular momentum
- The fact that a black hole cannot deform prior to colliding because of there inherent rigidity
The first item would be required for both black holes simultaneously. With other mass falling towards a black hole there is the possibility of radiation carrying off any excess mass, but this is not allowed with black holes as the . The second is required and makes it difficult to understand how the merging could physically occur.
More work needed, but see blog on 9 February 2016
Next, we suggest an answer to the question: where do supermassive black holes come from?
By User: Catsquisher (User: Catsquisher Transferred from en.wikipedia) [Public domain], via Wikimedia Commons
Included here because it is just a great picture.