Subwoofer to match the box measurements ?

The volume of the box is to match the resonant frequency of the speaker, which is the same for both speakers when you use two. The bigger the box volume, the lower its resonant frequency is. The speakers will produce the frequency range they are designed to play. If the goal is to emphasize only the lowest frequencies, go on and choose the largest cabinet that will work, but also consider the space available for this furniture. Perhaps, the physical size of the speaker frame require the configuration of the box must accommodate them. There's more to it than building the lowest frequency resonator. Just how much bass do you need, and how does it work with the rest of the system? To prevent a completely muddy sound which is just a honking rumble that overpowers the rest of the performance, the gain will need adjustment when you only consider the lowest end of the spectrum.

The recommended box volume is for a single sub. A box for two subs would need to be twice as large. A sealed box wouldn't require as much space as ported.

The 1.5volume ported is for one speaker so ifyou have 2 it doubles. When people do gain by ear they listen to the sub change tone. Facts by degree engineers