Reread your Post and you are talking about a Parallel AIO setup rather than a Serial setup.
What is the Make & Model of your CPU, GPU, and AIO that you are going to use?
NOTE: Found this article showing the two types of setup to cool both the CPU & GPU - Serial and Parallel: https://www.ekwb.com/blog/parallel-vs-serial-loop-why-choose-one-over-the-other/
From the article above showing how to a Parallel AIO Setup looks like:
Found this article which has much more details and diagrams on how to connect a AIO Parallel Setup for the CPU and GPU: https://www.performance-pcs.com/blog/dual-parallel-serial-loops/
Plus it shows all the parts you will need and prices to setup a AIO Parallel setup.
NOTE: It seems to be for multi-GPU Cooling but it should give you some idea of the part you will need.