you need to connect the ps-on wire to the ground or something like that or buy a relay, lol. Perhaps let someone more knowledgeable answer the question
I recommend you use the psu that powers mobo to power gfx too,use other psu for fans,h/d's,roms,prommies,etc.Also there is a relay you can get,but i couldn't find the link,This might help:- http://ecommerce.com.gr/didis/2psu1.htm