By "multiple ArcGIS Server sites" do you mean viewers/apps? If so, the answer is yes. Those can go on any webserver and are not constrained to being on the same machine as your ArcGIS Server instance.
If you mean multiple ArcGIS Server instances, I am fairly certain the answer is no. Your license is for a single instance. But this question is probably better answered by your account representative.
I suppose that you could have two 2 core servers (for a total of 4 cores--your license), but your question specifically says "same machine" and from a performance standpoint I am not sure how much that would gain you.
Admittedly, I may be misinterpreting what you are truly asking.
Thanks for the reply. I was referring to multiple ArcGIS Server instances.
I figured the answer was no, but wanted to ask anyways. I'll check with our account rep just to be sure.
Thanks for the info!