Well I don't know which of the coax you are using, but generally all these need a 2nd order low pass on the woofer, 2nd or 3rd order high pass on the tweeter section.
The 18" can be crossed low, because of the big "horn" (the cone). The 15" will probably need to be crossed over circa 1Khz-1.5KHz. That will go a long way toward taming the woofer.
There are still some thing you might want to trap out on the woofer, but just getting the right low pass filter on the woofer will work miracles for the sound. I know, I've done it, heard it.