You'd probably need more than one cotton ball to get a good seal over the powder. When I inquired about traditional wadding with tow, I was told one needed enough over the powder to pack down (firmly) to ~1/2". On the other hand, someone mentioned (on this board??) using a cotton ball quite successfully for an over-shot wad. I'm not sure if it'd work in a double, though, with the recoil loosening the second load.
Joel