HPS Shannon said:
Oh, the composite chart is pretty much a new and separate chart that shows the tone and entire chart of the relationship. A composite chart is a chart that is cats based on the time and date that two people first met. For example Aquarius was rising and Uranus was in the 8th house when we met so with that time, it shows the relationship theme and dynamic. So you'll have to figure/find out the time and date that you and your partner met.
There are 2 types of composite, one is the type you described, the other is taken using both natal charts and it calculates the mid-point of everything and displays as a new chart. So, if one person has AC at 10 Aries, and the other has AC at 10 Taurus, then their composite AC is 25 Aries. And so on with all their planets and house cusps. Astro.com calculates this, just select "composite chart" from the drop-down menu. This one is the official definition of composite, whereas what you described is usually called relationship natal chart.
I'd be interested to see which version seems more accurate in general for people, I've only had the opportunity to look at both for one couple as most couples do not know the exact time they met.