I am remodeling my master suite. The original floor was 1/2" play (randomly sized pieces looks like offcuts) with OSB over the ply. When we pulled the OSB up, the builder had tarpaper between the two layers. I am not a fan of tarpaper or OSB. We will be putting T&G ply down. What should we use between the layers of ply? FYI, we will be screwing the new layer down and then carpet.
Nothing. If you fasten the new layer well there's no need for it. Multiple layers of wood subfloor are commonly used without internal bonding or buffering.
If you're worried about sound due to movement of the random sheets, zigzag a 1/8" bead of construction adhesive over the original plywood to bond it to the new and help span any voids due to flatness issues.