Almost all PT wood these days is treated with copper. So I don't see the need for the copper above or below.

In your situation, I'd probably do something like concrete, capillary break (sill seal or something like it), PT bolted down (assuming there are bolts), timber sills screwed down to PT. If the bolts aren't there and you want to go through both sills, you could use longer expansion bolts but they're pricey.