Even though a column manufacturer specifies particular dimensions, the dimensions are not exact. This can cause problems when coupling columns in a single injection, dual-column analysis. J&W offers a column connection service, but if you want to do it yourself, try the following.
Connect the columns with the Y splitter and guard column. Verify the integrity of the connection. Heat the columns to a temperature at which you can inject a detectable, unretained compound.1 I like 150?C. Note the elution time of the compound. If it elutes more than 0.1 minute apart, cut 10-15 cm from the column with the later time. Repeat the process until the nonretained compound elutes within 0.1 minute on both columns, making the column's flow rates nearly the same.
Run a standard under typical run conditions until resolution criteria are met. Pick a member of a pair of compounds that is difficult to resolve on one or both columns. Raise the column oven temperature high enough so that it will elute the compound between 5 and 10 minutes, inject it and note the elution times on both columns.
When installing new dual columns for the same analysis, repeat the steps in paragraph two, inject the chosen compound and adjust the head pressure until the retention time for both columns is within a percentage or two of the previously recorded times.