In a smaller vehicle, the single sub will likely be louder. Larger vehicle, the pair. More cone area and motor force will overcome lack of power if you have the space to build.
adding a second sub, and dividing the power between tehm is a gain of +3dB Doubling the power to one driver is also a +3dB gain.
double the audible volume is a gain of +6 to +10dB, and requires an increase in power of 10 times the previous power level for every time the volume is doubled.
increasing power is an inefficient way to increase volume.
Yes, I agree that theoretically they should be the same. But theory breaks down in the real world.
For instance (given you have enough space to build), if you are running the 1 sub on 2k and it is reaching xmax or nearing xmech the 2 subs will be louder for daily driving. More coil in the gap and motor force will be more efficient.