this is the interpretation by Robert Hand (I copied it from the partner horoscope at astro.com, as Jude Law and me have an exact-Mercury conjunct in composite; of course I have no idea how it would play out, it is also part of a larger stellium, also including Venus, the nodal axis and with a bit more leeway Sun).
" The conjunction of the composite Moon and Mercury can contribute many strong and useful features to a relationship. Very simply, it means that the two of you can talk about your feelings instead of keeping them bottled up inside. The Moon conjunct Mercury may have two different kinds of effects.
If the lunar tendency triumphs, you will be able to discuss your emotional situations, but you will be very emotionally involved and subjective. On the other hand, you will really live and feel your emotions, so that something real is communicated between you.
If the mercurial tendency dominates, you will be more detached and rational but also less involved.
Obviously the best tactic with this aspect is to create some kind of balance between these two extremes."
I agree with that interpretation. If you put Moon and Mercury togehter you get a combination of the emotional, instinctual sphere and the rational, analytical one.
It can lead to communication and hopefully understanding about the emotions that are present in a relationship, but if it is losing its balance, it could also lead to being overly subjective and taking everything that is being communicated very personal, to an irrational degree even. ON the other hand it could also mean trying to rationalize and overanalyze the emotions in a relationship.
But what it definitely will do is bringing up an awareness and possibly communication about the emotional make-up of the connection.
However no single aspect can make or break a relationship, you always have to see what else is going on, but you knew I`d say that, right?
Personally I don`t think that this aspect alone can cause an emotional attachment; it is more about emotional awareness and communication. It is probably more likely that these people get attached to their shared thoughts, way to communicate and exchange of thoughts and ideas, as Moon can get attached to something, Mercury not very likely so, so Moon representing attachment, would get attached to the Mercurial side or feeling emotional about it, but that is actually more some kind of intellectual attachment, however as this is a shared horoscope, the mercurial side being emotionalized for both of them. this could pave the ground for some feeling of attachment, but only if other things come into play. In particular overlays to the natal chart. If someone`s Venus-Pluto-conjunction was falling onto that, yes then there would be a high chance that person would be getting attached and even obsessed with the way emotions are communicated in that relationship.
As for Draconic charts, I don´t think it can be transferred in a 1:1 way, even though there are common traits of course, but we always have to keep in mind that the Draconic planets always carry a nodal notion with it.
So if it is your Draco Mercury connecting to someone else`s Moon it is actually Mercury-Node to Moon, possibly indicating that the instinctual emotional expression of the one person is activating that growth potential of your Mercury. like you feel you have to learn something in terms of communication or communication skills and the other person by effortlessly and without thinking expressing certain qualities push you a bit into that direction.
Of course probably the result in an every day context will be similiar, as you will simply enjoy talking to them, about their feelings etc, to learn more about why they react the way they do, how they perceive the world etc.
But the difference is that once the Dracos are stimulated, there is a greater sense of meaningfulness and purpose, a "that`s where I want to go and what I want to develop" involved, while if that is happening in tropical, it is just that natural kinship and curiosity on both sides that pushes them into that kind of communication. Not unlike the exchange of children btw, (both Moon and MErcury have a natural relation to being childlike, because of their basic curiosity and innocence and just "being in the moment", be it from an emotional or instinctual sphere or a playful, experimental, discovering one).
* btw keep in mind the sign background, communication does not have to be VERBAL, in fact with Moon-Mercury a lot can be said and communicated without actually using words. Body language, other emotional kind of communication etc.
I suppose a Moon-Mercury in Gemini will prefer to talk a lot, but a Moon-Mercury in Capricorn or Pisces, probably not so much. The Cappy version might make just very necessary words, but these are important. And Pisces might more or less communicate telepathically with each other. Or develope a weird kind of language between them, almost like a code.
Also Moon-Mercury does not always have to be very deep, depending on what kind of Moon we are dealing with. Sometimes it can be awfully superficial, especially if Mercury gets the upper hand and trivializes the emotional connection.