Most 24-pin connectors have at least some of the wires doubled. These are the sense wires that tell the PSU what the voltage is at the end of the connector, so it can make adjustments. Cutting these (even if you crimp them back after) can result in incorrect voltages or even the PSU deciding to shut itself off.
The amperage limitation appears to be the connector overheating, as
Molex specifies 8A per 18ga pin in a double-row of 12, while the same gauge wire in the 2x2 P4 connector can safely deliver 10.5A. So more wires to the pin would not increase the safe load.