You cannot just combine the two rates as the two axles is a non commercial rate. Once you go over two axles you become a commercial vehicle and they put it to you. One exception is a SRW PU pulling a trailer if you pay cash and argue that you are not commercial. With dual rear wheels you are commercial on the turnpike even if only two axles.
If money is tight you could save some if you separated the car and had the DW drive it to get the non-commercial rate for two axles. Good luck with doing that for $5 less the fuel you use in driving the car.