Considerable improvement in heavy water separation efficiency in a thermal diffusion column is obtainable by inserting an impermeable sheet or permeable barrier in parallel to divide an open duct into two subchannels for double-flow operations. The analytical results developed and investigated using an orthogonal expansion technique are represented graphically and compared with that in a Clusius-Dickel column of the same size. The impermeable-sheet or permeable-barrier location effect on heavy water separation efficiency enhancement is also discussed.