Every roof i have torn off with 2 layers of wood on it has 3/8’s that has been delaminated to the point of no return. The second problem doing this is acctually hitting the studs because you must stagger the ply differently than the original wood, going over with the same pattern could cause problems down the road. Third problem is the weight of the added prlywood, many contracotrs i have noticed use a heavier wood on the second layer like 1/2 and i have seen 3/4 over 3/8. 4th problem to me atleast is it is just being lazy when you could tear off the original layer 1 sheet at a time and replace with new wood and not have any problems. To me it is cutting corners when you go over any plywood with another layer. Yes i know there is no code against it but when snow and ice combined with 24 inch centers and 2 layers of wood thats when problems can occur. Shake is a different story all together.
IMO it is not the way to go. You can do whatever it is you want but i feel better just doing things old school and not rushing. I will never cut a corner. And im a firm believer of 1 layer of wood on a home. Do whatever it is you want. Missing a stud will only make the new plywood in the cold stick up and curl slightly and then you have a water problem. I just dont believe in doing things that way.