How to add multiple buttons to TinyMCE?

First add your additional buttons inside the buttons callback..

function register_button($buttons) {  
   array_push($buttons, "quote","wpse-rules");  
   return $buttons;  
}

Then add additional buttons function inside the plugin javascript..

    init : function(ed, url) {  
        ed.addButton('quote', {  
            title : 'Add a Quote',  
            image : url+'/image.png',  
            onclick : function() {  
                 ed.selection.setContent('[quote]' + ed.selection.getContent() + '[/quote]');  
            }  
        });
        ed.addButton('wpse-rules', {  
            title : 'WPSE Rules',  
            image : url+'/image.png',  
            onclick : function() {  
                 alert( 'WPSE Rules!' ); 
            }  
        });  
    },

And so on, for any additional buttons you want..