That's what I was thinking - that at least one had to be a side kick - but it was late at night and I was sleepy. A split kick would be a side kick and a twist kick in opposite directions, but a twist kick is done with the ball of the foot, not the foot sword like a side kick, and it would be very hard to do 2 side kicks, simultaneously, to opposite directions, at least from a TKD point of view, unless you can do full splits in the air. If that's what you mean (side kicks in opposite directions, using the footsword, the outer edge of the foot), then the easiest way to describe it would be as if you were doing full Russian splits (the kind where your feet spread out from the center, rather than one leg forward and one leg back), leading with the outer edge of each foot.
If that's what you mean, then the best way to prepare for it is to work on Russian splits when warming up, because if you can't do it on the ground, you'll have real problems doing it in the air, and then practice jumping off both feet simultaneously, while kicking out to each side. If you're worried about the landing (and you should be), then the way to start practicing would be either jumping on a trampoline onto a soft surface or (better option) off the edge of a swimming pool or diving board, so you can get the kick down before you have to worry about the landing. The biggest problem most people have with kicks like this is not the kick itself, but retracting the kicks quickly enough to land on their feet.
If that's not what you meant, please let us know, and sorry for any confusion this may have caused.
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