It really depends on the application. From how I read your question, the aim is to switch between columns and not do a split dual-column analysis. Although it definitely does reduce time spent on changing columns, you should keep in mind that each run and temperature increase will wear out both columns. So unless you switch frequently, it may not be worth it by leveraging the assumed increase in expenses for column replacement. It also matters if you have dual detectors or not. If you are able to design a GC method that works for both columns/applications, i would consider dual inlet and dual detector (if running e.g. GC-FID). But it also comes down to the requirement for capacity/throughput.

Thanks Simon Gregersen Echers it is exactly a question of reducing time ,

but also our biggest issue is the leaks we got each time we do change the column however we go thorough all the necessaire steps but we got leaks and more frequently in the deactivated part , so we thought that may be the double column will reduce those leaks , according to your experience it really gonna help or it is just not related to our issue?

If you have LEAKS, then adding columns will not solve your problem. Solve the problem by correctly addressing the actual problem, not complicating it. You need training and support. Please hire someone with basic GC training to provide support and training to you. This will solve the problem and reduce the instances of leaks in your GC system.

Thanks William Letter , you are right , we are trying to do our best but sometimes it is really complicated with one GCMSMS to practice a lot of tests that requires each one of them a specified column , we tried to optimize the most of them on one column to eventually minimize the frequency of change but with some tests we have to change it each time .

If you have too many samples to analyze, then buy more systems or find other labs to run them. Perhaps a GC/MSD system would provide enough data for some of the applications saving you on the cost of purchasing additional MS/MS.

You need to make configurational modifications to your GC. A quick swap and capillary flow splitters are in use to perform backflush but also for dual-column applications for separated analysis purposes if two inlets/inj. ports are configurated within your system. Alternatively, using ultimate union (Tee) and connection between inlet and Tee using a deactivated short column (retention gap, consider like on column application or guard column) can combine two-column through one inlet/injection port. Without changing the column just plug one side of the Tee and permit to the solely one direction flow. At the end of the column, the abovementioned flow splitters can direct the column (in use) effluent to the selected detector. Due to the use of retention gaps before the analytical column, method optimizations must be utilized carefully and you should be aware of the well-pinpointed subject by Simon Gregersen regarding the same temperature gradients that will be operated for both columns even they are not in use which is dangerous in terms of bleed...