Whether both cores are used at the same time depends on the application you're running. Some support only one-core operations (a.k.a. no multi-threading at all), some, on the other hand, support only two cores and not four cores etc. This is not necessarily controlled by the running application only as well, sometimes, Windows manages the CPU usage itself. In other words - you can't influence that really.
About the index - Windows determines the index by the lowest sub-score. Mine, for example, is 5.1 (win7 Pro x64), but that's because my HDD is slow. If I would go and buy a new HDD, with higher capacity, faster access times and so on, my index would increase. Long story short - you gotta buy new hardware.