As long as you aren't mixing impedances and everything is in phase you probably won't physically
hurt anything, however I doubt you would get any advantage tonally.
As Isaac says it's probably not the best idea to share cabinets with different bass drivers with different excursions, response curves, etc., unless you are intentionally trying to cause distortion. I think that's why guitarists will "mix 'n match" drivers (or tubes even), to get an unique type of harmonic distortion.
Guitarists seem to use distortion as a regular tool, while bass players not so much. Of course there are examples of the opposite for both.
It can't hurt to give it a try, as Mel says don't forget the reverse terminals on the old JBL's.
Mike, picture a nice clean sine wave. Now picture adding another clean sine wave to that mental picture, same amplitude, same frequency but
reversed; when the first sine wave is peaking at the top the second one is simultaneously peaking on the bottom.
That very simplistically put is phase cancellation, the two sine waves effectively cancel each other out and the result is no sound.
That's
total cancellation, which doesn't happen much, unless you were to forget Mel's advice about the backwards JBL's.
There are varying degrees of phasing misalignment (using the basic principles of my total cancellation example) that can attenuate or boost a signal to plague (or benefit; like with a flanger or phase shifter) the player.
