As an animation graduate, I'm actually pretty damn eager to see exactly how they overcame this difficult task. I wonder how effective this method is for more intense interactions between the idols.
Generally speaking, you can guesstimate where each wants their hand to be for a relaxed high touch, then just take the generic animation you have for that high touch at that point in the dance, move the hand to where you want it to be, and figure out where all the other joints in your body should be to make that look natural. Work backward - start with your hand, move to your wrist, then the elbow, shoulder, then the hip.
Hugs, however, are a different matter. Holding hands can be like the above, but you have to take into account someone else's body shape for a hug and then the other girl has to conform to the hug, which opens up another can of worms to deal with...
... if they can do that with Takane and Yayoi and make it look as natural as Haruka and Makoto, I'll be impressed. Like, so much my heart would explode. Sadly, it's such a natural action, I think most people wouldn't see exactly how difficult it is to do try to recreate it.
