Twins Twist

Here's the real twist in a few years after they graduate from school he finds out they were using his brain to get a "A", while also getting a "D"

So I was looking at the Japanese version as a preview before this came out and its seriously one of the most confusing stories if you can't understand understand what you're reading.

The back and forth between the sisters was a little hard to grasp at first but it's actually pretty neat.

why are they polar opposites?