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Version 2.0 of Intel's ATX12V Power Supply Design Guide began recommending dual 12V lines for PSUs that can deliver more than 18A at 12V. In the latest guide, this recommendation has been reworded in section 3.5.7:
The 12 V rail on the 2x2 power connector should be a separate current limited output to meet the requirements of UL and EN 60950.
The requirements of UL and EM 60950 are related to safety. It stipulates that not more than 240VA is carried on any wires or exposed traces.