This is a bad idea. The 3-pin fan connectors are voltage controlled and the yellow wire carries a tach signal from the fan telling the controller how fast it's going so it can adjust the voltage up and down. With 2 fans connected you would at least have to cut the yellow wire from one of them and you would still very likely burn out the output power device that drives the voltage output since you would be pulling twice the current it was designed for (which is likely a few milliamps).
However, the CPU fan is PWM-based (Pulse Width Modulation) and that 4-pin connector can control up to 5 fans if they are designed for it as the Arctic Cooling PST fans are.
- D