that design only shows one woofer opening so i would assume that if you want 2 subs you can either build two boxes or find a different design that uses 2 subs
The cutout on the picture has NOTHING to do with the design. If that design was for 3 cubic feet tuned to 32hz and you had two subs that would work well in 1.5 cubic feet tuned to 32hz, that enclosure would hold both of them very well.
the tuning may be a little off depending on the displacement of the drivers. As you add drivers you will take net volume away from what they see, in turn increasing the tuning frequency.