  Dual active galaxy nuclei (dAGNs) trace the population of post-merger
galaxies and are the precursors to massive black hole (MBH) mergers, an
important source of gravitational waves that may be observed by LISA. In Paper
I of this series, we used the population of nearly 2000 galaxy mergers
predicted by the TNG50-3 simulation to seed semi-analytic models of the orbital
evolution and coalescence of MBH pairs with initial separations of about 1 kpc.
Here, we calculate the dAGN luminosities and separation of these pairs as they
evolve in post-merger galaxies, and show how the coalescence fraction of dAGNs
changes with redshift. We find that because of the several Gyr long dynamical
friction timescale for orbital evolution, the fraction of dAGNs that eventually
end in a MBH merger grows with redshift and does not pass 50% until a redshift
of 1. However, dAGNs in galaxies with bulge masses >10^10 solar masses, or
comprised of near-equal mass MBHs, evolve more quickly and have higher than
average coalescence fractions. At any redshift, dAGNs observed with small
separations (> 0.7 kpc) have a higher probability of merging in a Hubble time
than more widely separated systems. As found in Paper I, radiation feedback
effects can significantly reduce the number of MBH mergers, and this could be
manifested as a larger than expected number of widely separated dAGNs. We
present a method to estimate the MBH coalescence rate as well as the potential
LISA detection rate given a survey of dAGNs. Comparing these rates to the
eventual LISA measurements will help determine the efficiency of dynamical
friction in post-merger galaxies.
