The Q-FFF Theory suggests, that two equal sized SMBHs have preference!! over more different SMBHs BHs. After the reduction of the numbers of Super massive Black Holes a Dead Galaxy seem to be left over.
These equal sized BHs. do not merge with a bang, but form a quiet dual Herbig Haro system with two Polar GABHs ( Galaxy Anchor Black Holes.: or SABHs Stellar Anchor BHs) this is also observed by the so called "the final parsec problem" keeping always a certain distance between them. ( by a pushing plasma bar. see M87,)
The Plasma Bar, is the origin of the parsec distance (conundrum solved) between two Fermion repelling equal sized new physics Black Holes.!! see the Penrose-Q-FFF BH))
However, If such dual BH spiral galaxy merge with larger galaxies with larger BHs these Bhs form Stellar clusters at the inside or outside of the larger galaxy. ( see the stellar clusters of the Milky Way)These merging central black holes seem to change often into a tandem of two equal sized BHs after forming a TRIPLE BH system by a so called INTRUDER BHs. from outside, creating a small dead star galaxies between the these temporal three central Bhs, As soon as the three BHs are reduced to two equal sized Bhs by a merging process, see Image below, THE MASSLESS GALAXY MAY LEAVE TEMPORARY BEHIND. The Galaxy can grow further and most stars will be ejected into the central jets or Fermi Bubbles.
Dead Galaxies are also observed inside giant Galaxy bubbles assumed to be created by the same triple (or multiple) Black hole configurations around the observed local Dead galaxy. ( image below) We observe 4x and 3x 2x hotspots I suggest, representing the reduction process from 4x3x into 2x)