If both ridges are completely separated from each other in the attic than yes it is fine to ventilate both of them. If not then only vent the higher one. The reasoning is that they say the they are both vented the lower ridge could cause a "short circuit" in the ventilation system and cause it to not work to its full potential. To be honest I am not 100% I agree with that but that's how most manufacturers call for it to be done so that's how I do it.
Not sure exactly why you are asking about box vents on the back. Are you saying there is no ridge to vent that section? If so then yes, box vents would be an option. To work they also need an intake vent at the bottom of the section. If you posted some pics I would be better able to understand your situation.