Review on 🔌 Easycargo PWM Case Fan Splitter Cable - Dual Power Adapter, Duplicator Converter, Y Cable (4-inch 1-Pack) by Tim Nicholas

When using a 4-pin fan splitter, the 3rd pin is the speed pin. This allows your computer to read the fan speed. You miswired both sets of 4 pins. One should have 3 leads, the other should have all 4 leads soldered. Having both fans send RPM data back will result in false readings that will drive any RPM control software crazy and useless. In the worst case, you may end up reading almost twice the actual RPM used, which can be misleading as to the cooling you're getting and can lead to overheating of the components.