Cooling from more radiators gives diminishing returns. Depending on how your case is setup, using exhaust air from one radiator to cool another (ie if one radiator is intake and the other as exhaust, you lose efficiency.
My recommendation is, if you have the extra radiator, put it in on your next water change. If you don't, don't spend the money unless its burning a hole in your pocket. The extra radiator will help you run fans at a lower speed.
My setup is a 360, 280, 240, and 120; literally everywhere that can mount a radiator has a radiator. All radiators drawing air in so they all are running at close to maximum efficiency and a 250mm fan mounted on the side panel runs as exhaust. I can run my fans around 800RPM's so that only my pump noise is readily noticeable and cool an 8700k and 1080ti rather well. CPU reaches 50s - 60s when gaming and the GPU rarely goes over 32C; generally not much more than 7-10* over ambient.
Asus Prime Z370, Intel 8700k @5.0Ghz, Nvidia GTX 1080 Ti, 16GB @3400 CL14, Watercooling: x2 MCP355 pumps, x1 280 on front of case, x1 360 on top of case, x1 140 on back of case, x1 240 on bottom of case