I have noticed that certain points in the chart gather a conglomerate of asteroids.For example, I have such a stellium in Gem in my 5th house, around my SN/Karma: Kaali, Ishtar, a bunch of Vedic etc. (I will list all of them).
My partner has almost all his Vedic asteroids grouped in a stellium in Taurus the 8th house.
He also has a similar conglomerate around his Mars.
I think this will give a strong activation of that house around that point and a theme (in my case, apparently a fated, incarnation theme, in his case, it seems to be a Tantric theme).
In synastry, apart form the direct contacts, it will emphasize the connections related to those houses/themes.
The same could happen in the composite, of course.
What are your themes and how are they activated in synastry?
I will get back with my examples
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I seem to have loved you in numberless forms...