Kevin, on further reflection I am willing to admit that if you were to use a device such as the SVS AS-EQ1 (which performs EQ in both the frequency and time domains) to individually equalize 2 subs, it is quite possible that their actual summed output in a typical room will come close to their mathematically summed individual responses. Again, it remains to be seen whether doing this prior to equalizing the subs' combined output will confer any advantage (and I won't be doing it for all the reasons mentioned) but if this is indeed your intention, it's quite possible you will come close to accomplishing your goal.

Keep us posted.