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Generally speaking, CS2 may not utilize both cores at all times, but you should certainly see increased activity on each CPU when viewing the Performance tab of Task Manager. Some processes can use both CPUs, some do not per my understanding of what has been said here over the years.
An easy test is to open a reasonably large file and apply the Radial Blur filter with options of 100/Zoom/Best. That will take enough time on most systems to show loading of both CPUs near 100%. A popular "benchmark" from the RetouchPRO forum is to create an 8×10 inch image at 300ppi, fill it with 100% uniform monochromatic noise, then perform that 100/Zoom/Best Radial Blur and time how long it takes to be applied.
