The composite chart is created by taking the midpoint between pairs of each of two person’s natal planets (in this case Mercury and Neptune) and using them to create a third, new chart. The composite chart is not the same as synastry, which examines how one person impacts the other. Instead the composite chart represents the relationship as a whole giving insight into the dynamics upon which it, as a relationship, functions.
Composite Mercury symbolises the manner and method in which communication occurs in the relationship. It also suggests something about the nature of the decisions that you, as a couple, take. Whilst it is desirable, especially in an emotional relationship, to achieve some level of unspoken understanding, it is also necessary to actually speak to each other in concrete, straightforward terms. Unfortunately, many individuals party to a longer term relationship think the other person should implicitly feel and/or understand what is on his/her mind.
When this happens, it results in each dealing with the other as if she/he were little more than a product of his/her own imagination. This is especially the case when composite Mercury is conjunct composite Neptune. Although Mercury is meant to represent the workings of rational minds (precision, orderliness, and logic), when Neptune is involved, confusion is usually the order of the day. This is because composite Neptune tends to make individuals involved feel like soul-mates; caught up in an eerie sense of fated-ness, a deep (cosmic) union featuring redeemer, redemption, and spiritual awakening all wrapped into one.
However fabulously wonderful this might at first feel, often enough it ends in chaos and pain. Self-delusion of one of the greatest risks of a Mercury-Neptune combination. Initially it manifests in idealisation – how clever and funny and intellectually gifted is my other half – he/she is the perfect friend, companion, and guide. Naturally such idealisation is nothing more than illusion which cannot continue indefinitely; disillusion is all harder to take when reality finally sets in.
Whilst Mercury conjunct composite Neptune can preserve the magic in everyday communication, it can equally manifest in unviable dreams. Never forget that with composite aspects, nothing is one-sided; both parties contribute (equally or unequally) to the action.
Advice: From time to time, rationally test the real viability of the long-term goals set for this relationship. Pay as much attention (if not more) to what each of you actually does as to what you say. Don’t be afraid to demand straightforwardness and honesty. Someday the truth will out and no one wants to be caught off-guard.