You're looking for a lhd or left hand drive hub, haven't seen one in a while but that's the term you're looking for. I can't recall on the few I've seen if the right side of the hub was opposite thread allowing for a dual chain drive or if it was just a lhd flip flop hub which would seem to make more sense. There would be some style factor in running two chains like this but otherwise a pointless device, they would have the be the same freewheel and chainring on both sides or only one is working.