Don't do it. Use one PSU as there's nothing in the +5V side of the system that was designed to take power from different PSU sources concurrently. You'll almost certainly have voltage differences between the two PSU's and that'll probably result in pushing a current in ways not intended
If you want to push current through the original connector, splice an A1200 PSU cable onto your ATX PSU. Theoretically, this shouldn't be needed since the mediator supplies power to the motherboard through the edge connector and to any accelerator card too. Nothing left on the motherboard itself should draw that much power to really need it.
