One advantage is that any air cavity in the upper box will be at a higher frequency than if you had only one large box. And as Scott suggested, it can also help suppress half wave resonances in a tower like box.
dave
dave
Closed or controlled leak
Someone may want to create a subwoofer that is aperiodic, and the thought about losing some low frequencies by acoustic short-circuit may be bothersome. A subwoofer in a closed enclosure would be an obvious example: No leaks but good damping is desired.
On the other hand, a leaking resistance enclosure is useful creating a subcardioid or cardioid sound radiation. That may be advantageous in mid frequencies. Gradient 1.4 is one well known example.
Someone may want to create a subwoofer that is aperiodic, and the thought about losing some low frequencies by acoustic short-circuit may be bothersome. A subwoofer in a closed enclosure would be an obvious example: No leaks but good damping is desired.
On the other hand, a leaking resistance enclosure is useful creating a subcardioid or cardioid sound radiation. That may be advantageous in mid frequencies. Gradient 1.4 is one well known example.