What happens when an Ultra Massive Black Hole cannibalize another?

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.