You don't add the two cores together. Imagine that both cores are little elves.. Both elves can be working on two different things and be very proficient. But if you have them doing they same thing the might get in the way of each other, bumping into each other, whatever. But if you had a toy that was made for two elves to work on then you can almost add the cores together.. In that 2 cores working on one multi-threaded application can possibly see double the performance increase.
BTW i'm not making fun of you, If anything I'm making fun of my imaginative ass.
To answer one of your questions, the X2 6000+ runs at 3.0Ghz, not 1.5Ghz. But as others have said in more detail, they speeds arent added together by the number of cores.