I about to buy a new psu and I have some doubts, Is it the same 34 Amp in a single 12v rail that 17 Amp in each double 12v rails? Will both feed the same way a gpu that requires 20 Amps? Thanks for answering!
Either one will have no problems feeding a 20amp card. Its a tricky question and as far as I understand it it has to do with quality/reliability and that single 12v rails are somewhat better than split 12v rails. I may be wrong however.
Any quality power supply it won't matter if it's dual or single rail design.It used to be the single rail was better but design improvements have negated that advantage.
The single rail always was a gimmick, probably with a high-overclocked multi-gpu system you can trip the OCP but is very improbable. I build multiple system with Server/Workstation PSU converted to ATX, those PSU have multiple lines of 18A / 20A and never had a problem even with overclocked system.
Two rails with 17 A each don't total to 34 A so it's not the same as single rail with 34 A. Look at the total wattage from 12V rails and divide it By 12 to get the total combined amperage. In theory you could have issues with 17 A per rail with a 20 A card as likely just one rail is used for PCI-E power.