Neptune in 9th House synastry relationships are based on mutual interest in spiritual mysticism, education, culture, intellectual self-improvement, love for travel, and other types of cognitive activity, such as lucid dreaming.
With the positiveness of Neptune in this synastry, the “Neptunian” personality helps the Ninth House one to rely more confidently on direct intuitive experience in the search for spiritual self-improvement, and that helps the “Neptunian” partner to be more open in matters of inner spirituality.
If the synastry is negative, the partners instigate each other to get unreasonably infatuated with dubious mystical cults. The personality of the Ninth House considers the “Neptunian” nature impractical, mired in the delusions of their spiritual ideas. The latter, in return, considers the personality of the Ninth House to be too orthodox and intuitively undeveloped. As a result of such a mutual misunderstanding between Neptune in Ninth House synastry partners, the harmony is deeply disturbed, which in no way contributes to any kind of constructive life activity either in business or everyday life.
from F. Sakoyan, L. Ecker
Neptune in 9th House Synastry Explained
Neptune in 9th House synastry partners enjoy listening to your travel stories and meetings with extraordinary people, but at the same time, their feelings may be very different from those experienced by you.
If a partner’s Neptune is strong enough, his appearance in your life can be quite an experience for you, similar to immersion in a fairy tale: here comes a sorcerer who can perform miracles. It may seem to you that a little more effort on this partner’s side will make a mysterious esoteric country reveal its secrets to you; but, alas, it is not so simple, and a Neptune in Ninth House synastry partner is only able to show you such miracles from a distance. However, this can still mean a lot to you: at such moments, the voice of your inner teacher (or, in other words, the karmic egregor which shows the best direction of your further life path) can ring inside you … but you still have to go through it yourself (which does not mean alone).
At a low level of Neptune’s evolutionary development, you’ll constantly feel some kind of unpleasant deception: as if your partner all the time promises to show you the entrance to a magical land, but leads you nowhere. Or he can play the role of a Hierophant, initiating you into the esoteric art of hard-drinking or drug meditations, taking upon himself very heavy karma, but also strongly tying you to himself with a common destiny in the worlds of a low astral plane — a dangerous undertaking indeed which most people would prefer to avoid had they known what they are binding themselves to.
from A. Podvodny