An aerosol-generating system comprising: a cartridge assembly comprising: a cartridge assembly outer housing at least partially defining a mouthpiece having a mouthpiece air outlet; a cartridge at least partially positioned within the cartridge assembly outer housing and having an upstream end and a downstream end, the cartridge comprising: a first compartment having a first air inlet at the upstream end of the cartridge and a first air outlet at the downstream end of the cartridge; and a second compartment having a second air inlet at the upstream end of the cartridge and a second air outlet at the downstream end of the cartridge; a mixing chamber extending between the downstream end of the cartridge and the mouthpiece air outlet; and a ventilation air inlet extending through the cartridge assembly outer housing and positioned downstream of the cartridge, the ventilation air inlet providing fluid communication between an exterior of the aerosol-generating system and the mixing chamber; an aerosol-generating device comprising: a device inner housing defining a device cavity for receiving an upstream end of the cartridge; an electrical heater for heating the cartridge when the upstream end of the cartridge is received within the device cavity; a power supply; a controller configured to control a supply of electrical power from the power supply to the electric heater; and a device outer housing; wherein, when the upstream end of the cartridge is received within the device cavity, at least a first part of a downstream edge of the device outer housing abuts at least a first part of an upstream edge of the cartridge assembly outer housing so that the cartridge assembly outer housing and the device outer housing form a system outer housing, wherein the aerosol-generating system is configured so that at least a portion of the device inner housing is received between the cartridge and the cartridge assembly outer housing when the upstream end of the cartridge is received within the device cavity. a cartridge assembly comprising: a cartridge assembly outer housing at least partially defining a mouthpiece having a mouthpiece air outlet; a cartridge at least partially positioned within the cartridge assembly outer housing and having an upstream end and a downstream end, the cartridge comprising: a first compartment having a first air inlet at the upstream end of the cartridge and a first air outlet at the downstream end of the cartridge; and a second compartment having a second air inlet at the upstream end of the cartridge and a second air outlet at the downstream end of the cartridge; a mixing chamber extending between the downstream end of the cartridge and the mouthpiece air outlet; and a ventilation air inlet extending through the cartridge assembly outer housing and positioned downstream of the cartridge, the ventilation air inlet providing fluid communication between an exterior of the aerosol-generating system and the mixing chamber; a cartridge assembly outer housing at least partially defining a mouthpiece having a mouthpiece air outlet; a cartridge at least partially positioned within the cartridge assembly outer housing and having an upstream end and a downstream end, the cartridge comprising: a first compartment having a first air inlet at the upstream end of the cartridge and a first air outlet at the downstream end of the cartridge; and a second compartment having a second air inlet at the upstream end of the cartridge and a second air outlet at the downstream end of the cartridge; a first compartment having a first air inlet at the upstream end of the cartridge and a first air outlet at the downstream end of the cartridge; and a second compartment having a second air inlet at the upstream end of the cartridge and a second air outlet at the downstream end of the cartridge; a mixing chamber extending between the downstream end of the cartridge and the mouthpiece air outlet; and a ventilation air inlet extending through the cartridge assembly outer housing and positioned downstream of the cartridge, the ventilation air inlet providing fluid communication between an exterior of the aerosol-generating system and the mixing chamber; an aerosol-generating device comprising: a device inner housing defining a device cavity for receiving an upstream end of the cartridge; an electrical heater for heating the cartridge when the upstream end of the cartridge is received within the device cavity; a power supply; a controller configured to control a supply of electrical power from the power supply to the electric heater; and a device outer housing; a device inner housing defining a device cavity for receiving an upstream end of the cartridge; an electrical heater for heating the cartridge when the upstream end of the cartridge is received within the device cavity; a power supply; a controller configured to control a supply of electrical power from the power supply to the electric heater; and a device outer housing; wherein, when the upstream end of the cartridge is received within the device cavity, at least a first part of a downstream edge of the device outer housing abuts at least a first part of an upstream edge of the cartridge assembly outer housing so that the cartridge assembly outer housing and the device outer housing form a system outer housing, wherein the aerosol-generating system is configured so that at least a portion of the device inner housing is received between the cartridge and the cartridge assembly outer housing when the upstream end of the cartridge is received within the device cavity.