User blog comment:Plainblack/Blackbeard, the Enigma/@comment-6920514-20130903050139

I've always been partial to the theory that Teach has a parasite in/on him or has a conjoined twin attached to him. Just a convenient loophole to the 1 DF per user rule. As far as the tarp goes I'm sure he would want everyone to think he's got some special trick that they think they could do if they only saw how.

Second theory, maybe a little less out there is that he rescued an unseen crewmember from Impel Down that is content with living in BB's darkness allowing Teach to use those abilities when necessary. Or even just for that demonstration at Marineford since there wouldn't likely be many who would force him to use the 2 DFs anyway.