Typically the harness has to be removed so the booster weighs less (it's not really supposed to weigh more than 9 pounds...those extra harness ounces might push it over...). But beyond that, I think the possible forces that could be exerted into the chest from both systems could be injurious. My instructors told me that using both was a great idea if you had a young/wiggly/heavy kid, though (I tried to argue them out of it...)... At Lifesavers they just wouldn't even justify the question with an answer, saying it was just SUCH bad misuse they wouldn't even tell us what *might* be bad about it...(I suspect they haven't tested it, honestly)