Synastry overlays two birth charts and reads the angles between them — your Mars square their Moon, that kind of thing. A composite chart is different: it takes the midpoints between every planet in both charts and builds a single, third chart that represents the relationship itself, as if it were its own entity.
Astrologers argue about which is "more accurate." Synastry feels more personal because it keeps both people visible. Composite feels more elegant because it produces one chart instead of two overlapping ones. Both are legitimate methods; neither has ever been shown to predict something as granular as whether two people will want a second date.
What they're actually good for is description, not prediction. Synastry tells you where two specific people rub against or lift each other. Composite tells you what kind of "entity" the relationship becomes once it exists. Unfate leans on synastry-style overlays for the six fates because it keeps the individuals visible — useful when you're deciding whether to meet someone, not analyzing a relationship that already has years behind it.