The invention relates to a string machine for the tobacco processing industry, concretely relates to a multi-string machine for the tobacco processing industry, especially a double-string machine, especially a method of operating a filter nozzle double-string machine, wherein many strings to be manufactured respectively transfer a packaging material slice to a smoke gun mechanism preferably by a preset transfer speed and a preset tensile stress, wherein the packaging material slice is transferred to the smoke gun mechanism respectively in aid of smoke gun belts driven by smoke gun belt driving devices. The method is improved by the following methods: namely before the packaging material slice enters into the smoke gun mechanism, the tensile stress of the packaging material slice is detected by a common stress sensing device and used as the total tensile stress, wherein the corresponding smoke gun belt for each packaging material slice is transferred to the smoke gun mechanism in aid of te common smoke gun belt driving device for the smoke gun belt to the smoke gun mechanism and the common smoke gun belt driving device is adjusted by the total tensile stress depending on the tensile stress of the detected packaging material slice.