A tandem electrolytic cell with the structure [cis1, upstream nanopore (UNP), trans1 = cis2, downstream nanopore (DNP), trans2], an exonuclease enzyme attached to the downstream side of UNP, and a chemical adapter in or a profiled voltage over DNP can be used with bandwidths of a few kHz to sequence bases in ssDNA in natural order with high accuracy and without loss in trans1/cis2 or regression into DNP from trans2.