You need to post Brand and Model numbers of the AC units. Some OEM's installed 2-- 13.5 and some installed a 13.5 and a 11k and some installed a 15k and a 13.5 and some installed 2-15k units. Regardless of what another poster stated, there is a BIG difference between having that extra 1500 BTU's from a 13.5 to a 15K on any system. I have found that if your front unit on your configuration( the rear not really in the rear but amidships) you can swap the front to the rear IF the front is a larger BTU than the rear to get more accurate distribution to the rear. Once you post the Model numbers and we can then know the BTU's of the units you can get better help. Doug