It only "pretends" to have two cores ;
"Hyper-threading is where your processor pretends to have 2 physical processor cores, yet only has 1 and some extra junk.
The point of hyperthreading is that many times when you are executing code in the processor, there are parts of the processor that is idle. By including an extra set of CPU registers, the processor can act like it has two cores and thus use all parts of the processor in parallel.
When the 2 cores both need to use one component of the processor, then one core ends up waiting of course.
This is why it can not replace dual-core and such processors."
source cpu - Hyper-Threading and Dual-Core, What's the Difference? - Super User
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