I suspect you put the superhub in modem mode (which disables the wireless on it among other things) and then plug the additional router in to one of it's ethernet ports. Then everyone connects to the wireless on that router. So the Coax still only goes in to the superhub.
You may have to set the additional router to have it's gateway as the IP of the superhub. Not 100% sure but that should be all that is needed.
P.S: Hope I make it out of this thread alive, seems dangerous to post here