Ok to double plywood under flooring?
Leveling things would be first.
I am a big guy so I like a thick floor it is fine to add but remember you will need to trim molding at doors.
When you lay the new layer make sure to offset the seams, this will make a much better floor. Offsetting is making sure the seams are not on top of each other. If full sheets are used it can be as simple as cutting the first sheet in half.