It may be okay if you filter out the low frequencies running to the crates speakers with a passive crossover that you install inside the cab then continue to use the 18" cab in conjunction with it to fill out the bottom end. I'd look for a highpower subwoofer designed crossover of the same ohmage that crosses over in about the 100hz-200hz range and wire just the high frequncy out termanels to the speakers. The only shortcoming to this is not being able control the individual volume levels going to each cab but it should at least filter out the lowest of frequencies that will be most harmful to your guitars speakers.
Alternatively, look into just upgrading your amp to a biample one that will better accomplish the same thing or maybie a just a used 2x10 bass cab would be a much better economical add on. The 18" BW equipped cab is definately worth holding onto though, those are really great bass speakers!