How do dual 12v rails work?

   So I'm in the market for a new graphics card and was considering either a gtx 660 ti or 760, but they require at least 30 amps of continuous power. So I looked at my psu specifications and noticed it had dual (2) 12 volt rails each with 20 amps. I know the suggested amperage takes everything in your pc into account, not just the gpu, but can the gpu pull amps from both rails if needed, or is it limited to just one?

My Power Supply:

Any feedback is appreciated!

There is no way in hell that a single GPU needs 30 Amps. At +12V, that would be 360W power draw. Even on two 8 pin PCIe connectors, at ATX spec, you can only pull 300W. With dirty rails, and other magical trickery, you can easily push upwards of 500W, but unless you're going to LN2 that GPU past 1600mHz, you don't need more than 250W or power, i.e., roughly 21 Amps. You should be perfectly fine.

Overkill is the best kind of kill. I don't think it is possible to have too powerful a PSU, and having to upgrade one is always a bummer. It never hurts, especially when you start adding HDD's, overclocks, etc.

Read the 12v rail parts.

Unless you are using something from mid 2000s,connecting your cables to the wrong spots or using a psu with lower than needed wattage any rail config is fine.