Seems to be a good idea. You would avoid the warmer water going to the GPU after cooling the CPU, by having two completely separate cooling systems. The two hotter components would not affect each others temperature.
Have you considered having a normal radiator in the middle of the circuit instead of having another reserator? Maybe it would be enough to cool down the water between CPU and GPU.
The liquid temperature rise from any given PC heat sured is a couple degrees, so the typical practice of stringing waterblocks in series should be fine.
I find that higher radiator fan speeds have a large effect on cooling. Remember that air has much, much less heat capacity than liquid. So a relatively small flow of liquid is highly effective at cooling things, but a large flow of air is needed to take that heat away at the radiator.
Since the Reservator is a passive radiator with no fan, then your system might benefit from having another one. That would get more radiator surface area in contact with air. Or you could try to put a fan on the Reservator, or change to a radiator with a fan. If you did put a fan on the Reservator, you could make the fan thermally controlled so it only came on when things got hot. But to use a fan with it, you'd really need to have a shroud around the Reservator to direct the airflow over the fins, like putting it into a tube with the fan on the end of the tube. That would kind of defeat the purpose of having a fanless radiator.