How to have more than one controllers in AngularJS correctly?

I am getting started with AngularJS, and as I understand, I can have different controllers for different sections of my web page. I am having the problem getting it work. I have two sections of my page and corresponding to each one ng-controller - JSFiddle. Only the section that come first works. For example currently, app1 works fine, but when I move it below app2, only app2 works fine. What could be wrong? Much appreciate any explanation regarding why this behavior and any links.