If you
a) don't mount them in the same box
b) change the frequencies, remember there is a slope to the frequency cutoff. The sub doesn't magically quit playing at 45 hertz and the others begin at 45 hertz.
See...the problem is (from what I think) is that you have two different subs playing the same frequencies. It is much easier to install one or the other. Installing them both, and sounding good is rather tricky due to its easy to have cancellation.
Although I somewhat doubt that most of the people who said cancellation would even notice it if they heard it. Without careful listening, I don't know if I could due to the narrow width of frequencies be cancelled.