There was a
discussion about twins on the traditional board.
It seems there is almost always an alpha and a beta twin, i.e. one who is more dominant and one who is more following. I don't think decans can explain that in many cases.
Also, think of conjoined twins such as Abby and Brittany Hensel who share the same body. The one on the right side is outgoing and the one on the left side is more shy. Decans can't explain that because they were born at the exact same time.
There's a theory out there that twins each take on half of the chart, the alpha takes on the masculine planets and the beta takes on the feminine planets. This could explain the alpha/beta phenomenon that occur in most cases in addition to conjoined twins. But it seems not a whole lot of astrological research has gone into twin studies so there's a lot to learn about how this actually plays out in life in regards to the expression of planets and their aspects etc.
*Perhaps if you are a twin or have twins, a study could be to look at transits to masculine/feminine planets and see if one twin is more affected than the other during these times.