You can use a sawzall with a metal cutting blade to saw the nails off between the boards.Unfortunately they are still nailed together with 6” ring shank nails.
I won't use ring shank nails because once they start working on the wood from drying, swelling, wind movement,the ridges on the nails work away the wood as the nails go back and forth wallowing out the nail hole.
I do use cement coat nails instead.