ATX power supply question

I'm worried about 18AWG yellow wire (of the ATX PS) being able to carry 11A. Since the 8 pin ATX12V power supply connector consists of 4 +12V and 4 GND wires, can I join 2 or more +12V yellow wires and connect to the Ramps board? ( And do the same with the black GND wires?). This would, in effect, split the current between the wires?

I will use a different (2nd) rail for the 5 V connection.

Thanks