  We investigate the simultaneous triggering of active galactic nuclei (AGN) in
merging galaxies, using a large suite of high-resolution hydrodynamical
simulations. We compute dual-AGN observability time-scales using bolometric,
X-ray, and Eddington-ratio thresholds, confirming that dual activity from
supermassive black holes (BHs) is generally higher at late pericentric
passages, before a merger remnant has formed, especially at high luminosities.
For typical minor and major mergers, dual activity lasts ~20-70 and ~100-160
Myr, respectively. We also explore the effects of X-ray obscuration from gas,
finding that the dual-AGN time decreases at most by a factor of ~2, and of
contamination from star formation. Using projected separations and velocity
differences rather than three-dimensional quantities can decrease the dual-AGN
time-scales by up to ~4, and we apply filters which mimic current
observational-resolution limitations. In agreement with observations, we find
that, for a sample of major and minor mergers hosting at least one AGN, the
fraction harbouring dual AGN is ~20-30 and ~1-10 per cent, respectively. We
quantify the effects of merger mass ratio (0.1 to 1), geometry (coplanar,
prograde, retrograde, and inclined), disc gas fraction, and BH properties,
finding that the mass ratio is the most important factor, with the difference
between minor and major mergers varying between factors of a few to orders of
magnitude, depending on the luminosity and filter used. We also find that a
shallow imaging survey will require very high angular resolution, whereas a
deep imaging survey will be less resolution-dependent.
