any reason why you wouldnt leave them separated? like having an individual H50 cool each of the CPU/GPUs? you probably wont see any difference in temps, just because your going to use the same rads.
if you want to have fun modding i would say the optimal would have a rad after each water block (ie cpu/rad/470/rad/470rad/red/cpu) but i bet cpu/rad/470/470/rad/rad/res/cpu would work just as well and they're be slightly less tubing clutter.
if you want better temps you would have to upgrade the radiator. theres alot of guides for modding the H50 on the H50/H70 page
i doubt you can reuse the liquid inside the H50 since they include just barely enough to fill it, and you would need more to fill the res too. speaking of which, if you reuse the H50 rads and use distilled water, your going to need some anti-corrosion additive. since the waterblock/pump is copper and the H50 rad is aluminum. this
stuff i got from this guide
which i thought was very informative. you need it because of something about "galvanic corrosion" or "mixed metal corrosion". its all in that one guide i linked. hope this helps