Can the Ni-NTA Superflow Cartridges and/or Strep-Tactin Cartridges be connected in series?

Yes, Ni-NTA and Strep-Tactin Superflow Cartridges can be connected in series.

The Union M6 female/1/16’’ male connector from GE (Code No. 18-3858-01) can be used for this, for example.