I think a case might be able to be made for a two way subwoofer system, but not two different subs in one enclosure. For instance, the massive drivers that have great deep bass output like the RE XXX and the LMS Ultra are real energy hogs, and are relatively inefficient at upper bass frequencies. Other lighter drivers are very efficient at upper bass but you would not want them to go for lower bass. Also, as Hsu argues for its MBM-12, the subwoofer locations in a room that work that work great for deep bass can sometimes leave massive gaps in mid and upper bass frequencies, and vice versa, so two subs tackling different frequency ranges can work with the room and not against it. Furthermore, the narrower of a frequency band an amplifier has to deal with, the more headroom it has overall, since the amplified signal isn't being inflated by a other frequencies.