Not sure where I found the text below and the person who wrote it probably isn't an authority on the subject, but it's about Twin Flame astrology. They're talking about the composite though, but I think you can apply it to synastry as well.
quote:
You are already reunited with your twin flame and you might find it unnecessary to focus on a horoscope for couples to tell you why this relationship happened, the goals you will have together and the dynamics you will share, but this is really helpful my friends, so please enjoy.Twin flame astrology signs are calculated by determining the mathematical midpoints between each flames points and planets, from when they were born, it gives you a new chart for you both as one.
So which transits or planets will be the most important with helping you understand your twin flame relationship? In most cases it is Sun square Pluto, the ascendant, Sun in zodiac, Jupiter in zodiac and the aspects of Uranus.
First of all, so much of our transformation together is about our karma , our old deaths, rebirth, depth, magnetic pull and discovery about ourselves and others. We need to be aware that when our Sun sign squares with Pluto, our relationships are put to the test by dualities brought up to cleanse and teach us our lessons that we get from each other. The magnetic pull from our souls and a repulsion from our ego's, fear, past etc, at the same time. You will probably match perfectly, but there will always be differences to overcome. This is the beauty of twin flames, it's a fast track to working through karma and clearing your path together.
The Sun represents your ego, as in "me, me,me". The Sun = 1 while 8 is related to Pluto. Together they make 9, which represents the end, the last cycle. You have a chance to succeed or fail, if you fail you repeat your life again, it is a great loss, as together as twin souls you really have a better chance together than apart to achieve freedom from the wheel of life.
We are karmic beings and will need to even out our karma to progress. It is our goal to overcome your own ego and merge together with your #twin again and find #love and peace, be fulfilled, fell bliss, care for them and love them as much as you love yourself. It is way better to complete it than to avoid the drama of working together and through it. You now have karma to solve together and you need to face it with your combined wisdom and courage.
Twin flames chart ascendant may be either the same or completely opposite.
The position of the Sun may indicate which month you will meet up with your twin flame the first time on this plane, you will recognize each other on a soul level.
The position of Jupiter may indicate the year which you will both physically meet, as you may of only meet on-line and have formed a loving relationship there first. Jupiter spends about one full Earth year in each zodiac sign and takes 12 years to do full circuit of them all. Jupiter brings good luck, it makes dreams come true.
And finally, Uranus which is well known for bringing the unexpected and forging new patterns, will play a role in the twin flames horoscope.
This could be Uranus in opposition to the ascendant for twins who are still learning how to co-exist together, and Mars sex-tile Uranus as well as some other aspects between Sun and Uranus to mark the dynamics between the two.