It depends on how 'not so good' the not so good sub is compared to the good sub.
Consider that if the BIC is just slightly worse of a performer (output, extension, distortion, etc) than the Hsu, it would likely be better to have two BICs rather than one Hsu, because properly set up it will provide flatter frequency response than a single Hsu or BIC, higher output than a single BIC, and lower distortion than a single BIC. Adding it up, two BIC could give better performance than a single Hsu.
Keep in mind that the "could" in that last sentence hinges on the BIC having performance close to the Hsu, and that is unknown. I have yet to see comprehensive measurements of any BIC subwoofer.