Remote controlling an ATX power supply
I would suggest that you investigate simply driving the PS_ON# signal to both ATX power supplies from the same source. The source would come from the motherboard in the box with the computer.

This diagram shows how it would be interconnected:



This will result in when the soft power switch in the computer subsystem is activated the PS_ON from the motherboard will activate to a low level and both power supplies will turn on at the same time. 

Not that at the disk box you would just connect the PS_ON to the ATX power supply. If there is some existing drive of that wire at the disk connections end of the cable you would leave it open.

This should work correctly for the sleep support and would even support turning things back on should there be a wake event at the computer board such as a mouse move, wake on LAN or a soft power button press.