How does splitting power cables work in ATX PSU?
The power supply is not splitting the current. There is a single 12V regulator delivering power to 4 * 12V pins and 4 * Gnd pins. 


No matter how many 4x4 cables they put in the harness, the aggregate current at the power supply must be less than the 100A specified. 

How much current is drawn for each cable is set by the power domain it's plugged into on the Mobo or GPU, the power supply is not controlling current to the current domains.