Two people, one chart. The composite chart is not either person's individual horoscope - it's the chart of the relationship itself, built from the midpoint of every planet. Shows what actually lives between you.
The composite chart is a third chart - the chart of the relationship itself. Each planet is calculated as the midpoint of the two people's planets. If your Sun is 10° Aries and your partner's is 20° Pisces, the composite Sun is 0° Aries. It's not "your" chart or "theirs" - it's the energy that lives between you. Used to understand what the relationship has created - a whole that is more than the sum of its parts.