From what I understand, both jacks should be hooked together. If one speaker fails open, the one across from it will go silent. Often you can still see it move but that's just from the air pressure in the cabinet from the other speakers that are still working. That's not to say that someone may have changed the wiring of the cabinet or there is a bad connection in the wiring. The thing to do is test the cab with a battery to make sure all the speakers are working.