Laying T&G ply sub-floor over existing 1/2" ply sub-floor. What underlayment should i use between the layers?
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.