The frequency of a CPU describes how many cycles are occuring every second. For example 2 GHz equals 2 000 000 000 cycles per second. Some CPUs need one cycles to perform one operation (e.g. add two numbers) but complex CPUs also perform multiple operations during a cycle (thats why a core i7 may be faster than a i5 whith the same clockspeed).
Both of you cores can work at 2 GHz independent from the other one.
So for the simple example of 1 cycle = 1 operation the two cores could 'add' 4 000 000 000 numbers per second. But the task has to be running on two independent threads (= the software has to support multiple cores). 'Most' software is just running 1 thread so the are using just one core, the second one can be used by another program though.
Modern CPU (especially Laptops) govern their speed in order to save power/reduce heat. Even when you are using the computer, the CPU is waiting on data from the ram or harddrive, so running at full speed would be a waste of energy. Even more so if the computer is just idling.
Your computer is fine and okay. Run a benchmark if you want to see how fast it really is.
See also this.