Err... you need to state what OS your referring to.
For example, XP home allows 1 PHYSICAL CPU. Now that one CPU can have more then one cores in order to operate. So you could run a single CPU that has 8 or as many cores it has.
XP Pro supports 2 PHYSICAL CPUs. So that can even give you double the amount.
Since you say 2, you must be running XP Pro perhaps. I'm not sure about the Vista Product line.