I think a 301 redirect is in order, not because of duplicate content but b/c it will concentrate all the link juice at either
or
, but not split it up between the two of them. Now there's no proof Google views both sites as being separate, but it never hurts to be on the safe side.
Try to avoid reciprocal link exchanges and go for ABC/ABCD links. And remember - ranking for the keyword "free porn" is one hell of a job.. you might be better off trying to rank for "free midget porn" and will probably make way more $$$.