You are not allowed to combine weight allowances. The reason for the limit is not to save on fuel, but to save on medical expenses for ramp workers who file injury claims due to lifting extra-heavy baggage.
Pack using space bags and pop a cloth duffel in on top of the big bag (make sure your large bag's total dimensions are not more than 62", though -- that will cost you an overSIZE fee, and there is no avoiding that one if your bag is too big.) When you are waiting to check the bags, open the suitcase and put a few of the space bags into the duffel, then check them both. At baggage claim at the other end, take a minute to recombine before you head for your transportation. Much less to have to move and keep track of.