this dosent work like that when you have more then one +12V rails.it depends on to which +12V rail his GPU is attached !!! either to 16A or 18A.If his gpu is attached to a 16A rail and after overclokcing gpu it will require more juice so it will NOT take from the seconf +12V which is 18A.the only possibilty was that if his power supply comes with only one +12V rail @ 52A or higher but only one +12V rail. if he force his psu to give more then 16A on 16A rail or 18A on 18A rail either his computer will turn OFF or power supply BOOooOOM !!!its got 3 12v rails with 16A, 18A and 18A respectively. which means 52amps in total.