The natal composite is a summary of the synastry contacts. It shows us how the natals blend when the two persons come together, how they react to each other in relationship.
This is a fixed. The natals don't change and the composite doesn't change.
The progressed natals, and the progressed composite, show how these two evolve over time. For example, they may have never looked at each other, but then the Venus of one progresses to the sun of the other and all of a sudden they light up and fall in love.
However, once that Venus moves on, they have to rely on the basic synastry, and if it isn't strong, the Venus energy wanes, and it could fall apart.
Just as the opposite could happen, the Venus sparks it off, but even when it moves on, there is enough between the two charts to hold them together.
So, if we see mars/uranus/moon in a t-square, we understand there is a powerful mechanism that occurs between these two people. And it will always occur, it won't go away.
Looking at the progressed composite, we understand further that they have moved forward in the relationship, but the underlying tension remains, mars/mercury/saturn is not easy to handle, and inconjunct obsessive dominating Pluto, shows the energy comes out in a different area, in a slightly different way but the pressure is still there under the surface.
How? Well, Neptune is now square those planets in tension. So evasion, deception, illusion are all playing a part and glossing over the negative, but this is all just illusion.
In the base composite chart several planets connect up, almost in the same degree: Saturn/jupiter/mars/venus/mercury/neptune/uranus/pluto/Chiron.
The unrelated sun/north node conjunction indicates that this relationship aims at finding and expressing the individual egos of the two, they each have to find themselves and express that in the relationship, and this sense of egoism will become stronger and stronger and more compelling as time goes by, because that is the purpose of their being together.
The tensions are difficult to handle, but we see that Neptune steps in to vaporise them, to allow them both to evade and look the other way.
So the irritation, the fighting, the dominance, the bickering still occur but they have a sugar coated escape hatch built in to help sustain it all.
The synastry and composite are always the most important, the progressions show the path they are following.
The best of a progressed chart cannot put into a relationship what is not there, it can only show moments when the tensions ease up.