So are you trying to change the content you are showing from A to B when they click (switch the content) or to keep adding it?
If you are switching you should just set those values with ng-bind so they update when you change them and override the content of A by B. If you are trying to add, I would put an ng-repeat list fetching from an array (ex. contentToShow) where you just push items each time they click a button.
Also had a look at your codePen, sorry but I couldn’t get much. The CSS tab is for styles, not for data, and I got lost in those controllers.