Kar has it right. A LC loop doesn't automagically lower temps to 40. If you're already under 70, a custom loop isn't going to get you much lower. A better waterblock than the AIO and a bigger radiator might get you a few degrees, but nothing else.
Also, while it is possible for a CPU and GPU to "steal" cooling from each other on the same loop, it takes either a poorly configured loop and/or components that generate a LOT of heat. Neither applies here. While the 6700K is higher-end, it still puts out much less heat than X99 CPUs and others. Also, GPU efficiency keeps increasing, meaning heat output is going down. Even SLI 1080s aren't a huge heat load. All this can easily be put on the same loop without any "thermal contamination," if you will.
However, as I said before, if you're really worried about it, use one loop with two radiators, each placed immediately after or before the CPU and GPU. That way, whatever heat the CPU dumps into the coolant can be dealt with in one radiator before going onto the GPU, or vice versa. If you're already willing to spend money on two complete loops, saving money on a second pump and reservoir will let you put that into bigger radiators and a better pump.