What happens when an Ultra Massive Black Hole cannibalize another? Say hypothetically an UMBH with 50 billion solar masses (supposedly upper limit) is beginning to merge with another (Super)MBH similar to Sagittarius A* that sits at the center of the Milky Way Galaxy, what would happen next in this clash of the titans?
From this article:  


  “Bigger black hole masses are in principle possible – for example, a
  hole near the maximum mass could merge with another black hole, and
  the result would be bigger still. But no light would be produced in
  this merger, and the bigger merged black hole could not have a disc of
  gas that would make light.”


The theoretical size limit has to do with the ability to form an accretion disk, not the hole itself.   While it's probably an extremely rare occurrence due to space expansion, if a 50 billion solar mass black hole was to merge with another 50 billion solar mass black hole, you'd have a 100 billion solar mass black hole.    There might be some interesting spiraling in towards each other and some gravitational waves made in the process.   It would be cool to study, but in such a scenario, there's no force that would prevent the two black holes from merging into a bigger black hole.