The Cortex-A15 is the heavy lifter here and will be handling all the power hungry tasks whereas simpler tasks will be handles by the Cortex-A7. The Cortex-A7 is designed to be very power efficient, which means the device will be using 70% less power while running on the A7. Think of it as the Optimus Technology from NVidia, where it switches to a low power GPU when not doing graphics heavy task, except here both the processors will be on a single chip.
Both the A15 and the A7 will contribute two cores to the processor so even though it will have four cores in total it will only be using two at most at any given time. The A15 will have a clock speed of between 1-1.5GHz and the A7 will be clocked at 1GHz.
But this new Exynos processor is expected to come out in the second half of 2012 and unless Samsung pushes the launch of the Galaxy S III till then it will have to settle for the recently announced Exynos 4212 with a dual-core 1.5GHz CPU that has 50% more powerful graphics performance and consumes 30% less power thanks to a new 32nm low-power process.