When taking a duplicate sample in soil vapour a y-splitter must be used, 2 canisters and 1 controllers (keep in mind that the sampling time will be doubled.
Connect the flow controller to the source, connect 2 summa canisters via the y-splitter.
Conduct your typical shut in test, leak test, close off the well head and open both canisters, then open the well head and begin sampling.