Here is a simulation to illustrate. This is based on a Hornresp simulation that has been exported into REW for comparison:
In light blue you can see dual Rythmik 12" servo subs in a sealed box with 1w input. Around 88 dB nominal sensitivity which drops to 80 dB at 20 Hz. The black line shows T20, a 20 Hz tapped horn with around 500L volume. The nominal sensitivity at 100 Hz can't be compared, however it is 89 dB at 20 Hz.
The dark blue line shows the excursion limited maximum output. Around 370w shared between the two drivers has the driver reach excursion limits. 106 dB max SPL at 20 Hz. The tapped horn increases sensitivity and controls excursion more so that the full 740w can be used and excursion limits are reached around that point. The output is 118 dB, which is a 12 dB increase in output.
So the result is that two 12" drivers with 370 watts each can achieve the same output as eight with twice as much power. Since drivers and amps are much more expensive than a few sheets of cheap ply, this works out as a cost effective choice.