I was curious yesterday about how would one go to create a mixed iPhone/Android project so I started looking for a suitable guide to do that. Unfortunately I found no such guide, so I wrote one my own.
You can find this guide here: http://gameit.ro/2011/08/creating-an-iphone-and-android-cocos2d-x-hybrid-project/
As I state in the article, starting from the HelloWorld project included in the cocos2d-x bundle is probably an easier alternative, but if you want to do this manually, now you have a guide to help you do that.
The gist of the article is that you first create an iPhone project, then the Android project, then move the files from the Android project over to the other one and update some of the paths.
Let me know if you have any questions.