What you propose does sound correct to me. I was unaware of the extra channel in the ICBM, but with the two subs, I think your plan makes the most sense. I think the result would likely be better than using the crossover in the NHT sub as I originally suggested. I'm guessing you would get a better db per octave curve from the processor.
Sounds like you've got some fun on your hands. Let us know how it works out!
S.