The first thing that takes place is that the supermassive black holes will move towards the centre of their new common system and eventually interact by forming a binary orbiting one another. The next stage is believed to lead to one of two scenarios, namely:
1: If the two black holes are of different sizes, then over millions of years their orbits will shrink with the black holes eventually merging to form an even bigger black hole with a mass of up to 5 billion times that of our own sun. One such occurrence can be observed in the constellation of Hydra, in a galaxy named NGC3393, where NASA’s X-ray telescopes Chandra is recording the gradual merger taking place 160 million light years away from Earth.
2: If the black holes are of an equal mass, however, as they come into contact a powerful recoil is believed to take place in which one black hole is kicked away at speeds of millions of miles per hour. It may even be ejected from its own galaxy leading to the potentially terrifying conclusion that, although rare, there are likely supermassive black holes travelling through intergalactic space which we have no way of knowing about.