A common head for use in combination with a cartridge assembly, the cartridge assembly being retained within the material containment unit of a dispensing device and arranged to discharge reactive components from larger and smaller cartridges in response to actuation of the dispensing device. The common head includes inlet openings and a rigid walled tube having a partition positioned therein to define larger and smaller pathways to enable passage of the reactive components from the cartridges into and though the common head in a predetermined volumetric ratio and prevent backflow of the components which can lead to unintended curing prior to dispensing. A flexible liner arranged for positioning within one or both pathways of the rigid walled tube is also provided to prevent such backflow and unintended curing prior to dispensing.