The synastry chart is the combined charts of the two natal(birth) charts. It is a wheel within a wheel. The composite chart is a single chart of the relationship. sort of like a natal chart of the relationship. . You know what is strange? When there is a wonderful synastry, but a poor composite. This happened with a friend, recently. The synastry showed that they were of one mind and soul. However, they broke up, after three months. I was asked to do the Composite. It was very weak, almost anemic.
There are different schools of thought among Astrologers as to the weight of each. Kim Kadashian and her ex husband had a poor synastry, but a wonderful composite. Perhaps, they did not take the time to settle in and see what the relationship would have brought. According to Astrology, it would have been good. In this case, the relatively poor synastry may have undermined what could have been a wonderful, long term marriage.