Welcome to the Board, and the forum i might add
. Most people who have reefs you use just live sand and have nothing under The substrate. I think you should just leave the sand the way it is, that is fine. If you did mix them together, some areas would have more dead sand, then live, and food/poo/pee wouldnt be disolved or eaten, leaving it to rot in the tank. I hope this helped.
From,
Your friendly neighborhood Handy man.