If plywood is used to raise the level of a concrete floor that is in poor shape, does anything need to be done between the substrates? The concrete has extensive mastic and carpet glue problems. The plywood will more or less just be covering that floor to make way for something else but I am concerned about air/bugs between the layers. Any advise is greatly appreciated.

Do not use plain plywood. A pressure treated tongue and groove plywood is needed for this installation. If the floor isn't pool table flat and level you may need shims or sleepers. I'd put down a 6 mil plastic vapor barrier under the plywood. You could secure with Tapcons or a Remington nail gun.

I Concure, That's What We Do.