Having two significators is only half the method. The other half is the list of conditions the tradition applies to each of them, and the list is the same for every planet in the chart, which is what makes traditional astrology teachable.
The questions are these. Is the planet in a sign it rules, exalts in, or is debilitated in. Is it in an angular, succedent or cadent place, which fixes how strongly it acts. Is it direct in motion, stationary, or retrograde. Is it free of the Sun's beams, or hidden under them. Is it aspected by a benefic, by a malefic, or by both, and does the aspect apply or separate. Is it of the chart's sect or against it.
Applied to Mercury and the Moon, that list produces a description of a mind which is specific without being flattering or unflattering. A retrograde Mercury is not a defective one; the tradition reads it as a mind that turns back over its own ground. A Moon in the beams of the Sun is not a damaged one; it is one whose impressions are hard to see because something brighter stands in the way.
The value in this for a modern reader is the discipline of it. Condition is established before meaning is assigned, and a description arrived at that way is at least consistent from chart to chart, which is more than can be said for the impressionistic method that often replaced it.