[en] The twin column concept is designed to enhance the field of application of front chromatography. Operating in that mode, we could demonstrate, that the concentration curve of the chromatographic column effluent is composed of two fractions, the wall and interior fraction, having each a breakthrough volume at C_BT/C_feed = 50 % for the interior fraction (V_BT) and C_BO/C_feed = φ_P/φ_C for the wall fraction (V_BO). The corresponding volume ratio amounts to π/(π+2) = 0.61. On this basis, we established a chromatographic array composed of feed tank - 1. column - detector - 2. column - product tank and loaded the 1. column, until 1.22 V_BT of feed has been conveyed through the column set. Then, CBO has been reached in the 2. column effluent. We can, thus, control the process below the detection limits of commercially available monitors. We employed the concept, to investigate the long-term behaviour of extractants recommended for the HLW partitioning. We could demonstrate, that CMPO appears to be an adequate extractant for partitioning HLW solutions in an actinide/lanthanide fraction and a fission/corrosion product fraction. However, we could not confirm, that (C1-C₆H₄)₂=PS₂H is sufficiently stable to maintain the chemical properties required for an actinide/lanthanide fractionation. We used one column filling with 10 g CMPO to process in total 1001 feed, thus economizing our studies. (author)