Catalysis of the reactions: n 3'-phosphoadenylyl sulfate + chondroitin 4'-sulfate = n adenosine 3',5'-bisphosphate + chondroitin 4',6'-bissulfate + n H+, and n 3'-phosphoadenylyl sulfate + dermatan 4'-sulfate = n adenosine 3',5'-bisphosphate + dermatan 4',6'-bissulfate + n H+.
3\'-phosphoadenylyl-sulfate:dermatan 6\'-sulfotransferase activity