Two-dimensional separations can be carried out in gas chromatography or liquid chromatography.  Various different coupling strategies have been developed to "resample" from the first column into the second. Some important hardware for two-dimensional separations are Deans' switch and Modulator, which selectively transfer the first dimension eluent to second dimension column.
The chief advantage of two-dimensional techniques is that they offer a large increase in peak capacity, without requiring extremely efficient separations in either column.  (For instance, if the first column offers a peak capacity (k)of 100 for a 10-minute separation, and the second column offers a peak capacity of 5 (k) in a 5-second separation, then the combined peak capacity may approach k × k=500, with the total separation time still ~ 10 minutes).  2D separations have been applied to the analysis of gasoline and other petroleum mixtures, and more recently to protein mixtures.