Then one day an astrologer shows you that the composite chart of the relationship with your best friend has a bold 1st house Aries Sun trine an adventurous 9th house Sagittarius Moon; and the composite chart with your romantic partner has an extremely comfortable, yet other worldly, 4th house Sun, Venus, Mercury and Jupiter in Pisces square Neptune in the first house. What relationship do you feel more comfortable with? We look to your natal chart for the answer. Perhaps you have a very cautious Capricorn Ascendant square a Moon-Saturn conjunction in Libra and you would rather not risk killing yourself climbing Mount Everest, nor do you want to escape to cloud nine every weekend. And in that case, maybe it's time to find a new lover and a new best friend.
On the other hand maybe with your conservatism (Capricorn rising), you are able to balance (Libra Moon) both relationships without going to extremes, and you, with the other person's cooperation, are determined to make both relationships work. Maybe it's my eternally optimistic Sagittarius Ascendant talking, but in my opinion there is no astrological configuration that says you can't. Awareness is the key.
I believe the synastry, that is the actual energy exchange between two people (shown in a relationship chart which uses the geometric aspects between the planets and points of the two natal charts) is the key to the composite chart. The best composite chart in the world is not going to work if the two people don't want to be in a relationship together and synastry determines how each person affects each other. And by the same token the individual natal chart and progressed chart are the keys to the synasty (relationship) chart. If the person's natural temperament or current circumstance is not conducive to a relationship with the other person, then it does not matter how strong the synastry tie is, the relationship will never happen.
Usually we see the major themes repeat themselves in both the synastry and composite charts for example if the two people overlay a lot of planets in each other's 4th house (synastry wise) you might also find that the 4th house is very active in the composite chart and of course that means hearth and home will be very important if these two people get together.
Also remember you create a composite chart with everybody on earth but it will only really come into play with a handful (or less) of people in your life at any given time. The composite chart is best used as a guideline after the two people have been together a while and are spending a lot of time with each other. Synastry is a better guide to the short term people in your life.
Once you get to that "composite chart worthy" phase in your relationship, it's probably smart to try to personify the composite chart if you are not already doing so naturally. If you have a composite 4th house Pisces stellium then getting responsibly high and listening to ocean sounds together in the comfort of your own home might be exactly what your relationship needs to stay vital, but if you have a composite 1st house Aries Sun with someone and you spend too much time together at home and asleep, that Aries temper is going to rear it's ugly head and the two of you will have to wake up and adjust yourselves or be angry all the time.
I've seen beautiful (lot's of trines and sextiles) composite chart couples break up after a short time and I've seen composite charts where there are lots of squares and oppositions where the couples stay together for a longtime or a lifetime. Of course a less challenging composite chart is more desirable only because the relationship as a separate entity has a life of its own with its own unique set of problems separate from individuals involved and has nothing to do with the way they feel about each other. The composite chart is sometimes perplexing to the individual people involved because it's this outside force that they can't control.
The way I look at it though is that I'm not going to give up on my life just because I have some challenging placements in my natal chart. They are the keys to growth. They make life challenging and interesting. So why would you give up on a relationship with someone you love just because the composite chart suggests some tension, and by that same token if you can't stand each other and the two of you are miserable together, and the composite chart says you're destined for eternal happiness, then this astrologer advises you to get the hell away from each other (laughing). Astrology is just a guide to help you become more conscious and hopefully make better decisions, that's all.