Generally you just add R-values. So if you have a NeoAir (R-2.5) and a RidgeRest (R-2.6) then you'd have a total R-value when stacked of R-5.1.
There seem to be two schools of thought on which order the pads should be stacked. Some people say you should put a thick air pad on the bottom (ie. NeoAir) and the CCF pad (ie. RidgeRest) on top, because the thick air pad loses heat through the side walls, so if it's on top then you get cold creeping in from the sides and that will affect you more, than if it's on the bottom. The other school of thought is that putting the thick air pad on top is better because it's more comfortable. This is the school of thought that I agree with. Maybe if I was really worried about being way too cold some night then I would try putting the CCF on top, but normally I don't find much of a difference so I go with the more comfortable setup.