Two D5's are absolutely not necessary unless you simply want to spend extra or plan on building a larger loop with more rads and blocks off this kit. Otherwise, the X20 750 pump is good for a CPU only loop or even adding a single GPU, DDC or D5 pumps would be suited for a larger loop. Dual pumps when you are considering CPU, 2+ rads, 2+ GPUs or you just want really good flow rates.