How to make game use dual core or duo core by Unity3D?

unfortunately it can not be done much in your side. you can create threads in your game and do any calculations in your threads but you can not call unity apis in threads other than the main thread. unity just do some of it's calculations in another second thread. those things are scene loading and downloading content in pro version. i think they can make physics and rendering systems multithreaded and do optimizations that make us able to use hyperthreading and multithreading in our games. you can request thiss in feedback or just find a request and vote for it.

You have to write your game multi-threaded. There are also asyncOperations for streaming content, this is mostly a pro feature