A cartridge assembly for an aerosol-generating system is provided, including a cartridge having upstream and downstream ends, a first compartment having a first air inlet and a first air outlet, and a second compartment having a second air inlet and a second air outlet; a cartridge holder, a portion of the cartridge being received within the cartridge holder; a mouthpiece having upstream and downstream ends, at least a portion of the cartridge holder is received within the mouthpiece, the downstream end of the cartridge is spaced apart from the downstream end of the mouthpiece and defines a mixing chamber therebetween, and another portion of the cartridge holder is spaced apart from the mouthpiece and defines an airflow channel between the cartridge holder and the mouthpiece; and a ventilation air aperture providing fluid communication between a downstream portion of the airflow channel and the mixing chamber.