The 12V rails in a power supply are powered by a common source, think of it as a well supplying water to a house. The source has a max rating and you can pull any amount up to that. The rails serve as hoses, you can draw up to 19A from a single rail before the system shuts down, but if you try to draw 19A from all three at the same time the source cannot keep up, it may only be able to do 40A total, so trying to pull the max from all rails will overload the source and cause it to shut the system down.
There are only a few power supplies that you can actually add the max rating per 12V rail together and get the total power for the 12V rails combined, most will be significantly less. If you see a 500W unit with 3 12V rails at 19A each you know it cannot be added together because thats 57A*12V=684W which just isnt right for a 500W unit.
Seems like it actually has 3 into one so should be plenty of power to run those cards.
From the jonnyguru review
This puppy sure has capacity on the 12V side of things, doesn't it? Almost the full output of the unit is available over there. And like I said, that whole 52A number goes to one, not three, 12V rails. 5VSB is a little on the high side, as not too many rigs would need 3A there, but it's cool to see all the same.
Graphics cards are always rated for the whole system. They only specify the 12V rail these days because 80% of the system power usage is from the 12V rails. A GTX 560 will not pull 30A off the 12V rail, a system that it is in might.
An Anec NeoPower 650W will be plenty for either of those two cards, its got a combined rating on its 12V rails of 624W(52A) so you arent going to run into issues with those cards.
Again, proof that those ratings are for the system as a whole. The card gets 75W(6.25A@12V) from the PCI-E slot, a 6 pin connector provides another 75W, and an 8 pin connector provides up to 150W(12.5A), and neither of those cards pull the full 300W limit for a PCI-E card so even if they got all of their power from a single 12V rail you would still wouldnt trip the over current protection.