astro.com states:
"The Ascendant in a composite chart can be computed in different ways, depending on the technique the astrologer uses.
One cannot look at the Ascendant in an isolated way without considering the position of the MC. We calculate the MC first and go from there. Sometimes a raw calculation of the Ascendant results in an impossible sequence of houses. A change in the birth time of one of the partners may have the same result. This is why the ASC / DESC axis is swapped (by 180°) in such cases.
With midpoints, there is always an axis with a near and a far midpoint for each pair of factors. Both midpoints are nearly equivalent in meaning and can be reversed if needed."

There are also problems with Venus and Mercury:

"There are different ways of calculating composite charts. One of these differences concerns Mercury and Venus: some astrologers transpose Mercury and Venus by 180° to the opposite sign, if they come out on "the wrong side of the Sun" in the chart.
We offer both variants. In the chart section of the AstroShop, for type 62, the two planets are always put on the side of the Sun, in chart type 671 and all reports they are not moved but remain where they technically fall."

You cannot do composite charts for twins, apparently, as well.

I have a few problems with the statement,"Both midpoints are nearly equivalent in meaning and can be reversed if needed" Really? Are they saying that moving a midpoint Venus position by 180 degrees makes no difference to the composite chart reading? Hmmmm.