A synastry is the comparison of two natal charts, the composite is the blending of these two to understand how the relationship itself works out and the Davison is a variation of this.
So, the aspects in the synastry are the basic exchange of energy, in the case of NN, each person's relationship themes and challenges. It is considered that the NN is where we are aiming for in this life, and the SN is what we carry over from the past. As we move along, the NN becomes stronger and the SN less so.
So, if someone's Venus conjuncts our NN it brings love, affection and friendship into our life, and the relationship between the two. How this acts out will be shown in the composite (and the Davison, which could vary from the midpoint composite).
The midpoints could show a different story depending on how the other planets line up, so planets in natal square in the synastry become trines in the composite, etc. Also in the natal the inner planets mercury and Venus are never more than a sign away from the sun, while in the composite they could be anywhere.
These are "pictures" of energy. They help us understand what we have on hand and how it all blends and therefore how we can work with it.
This does not mean that a specific outcome is guaranteed. It is we who determine it.
Sometimes things will go ahead, even in a very positive synastry between two people, and then just end, because neither needs the relationship anymore, and they move on.
Sometimes the energy is just too negative to deal with, and it ends for this reason.
There is no reason why a Venus/Node contact should inevitably end, or why it should necessarily continue. It is part of the larger picture.
If, for example, you say someone's Uranus is conjunct your NN, then chances are the contact would be sudden and also not long lasting, only because Uranus's energy is like that, it is easy come easy go. But if there are other lasting contacts between the charts, like strong Saturn ties, the relationship could remain and the Uranus energy would express itself in other ways, for example being an open relationship or both partners being involved in astrology!
So really, it is up to us.
Consider both natal charts, what each person seeks and how their patterns match up with what they both want and need. Then check the composite to see how it manifests together.
And don't think that one aspect can be a make-or-break of the relationship. One brushstroke doesn't make a painting, it's the whole together that matters.
