How to return true after function is comopleted?

i know its elementary javascript question but still it'll be of great help. i'm stuck in following function:

Utils.get = function (action,index,callback){
    var books  = Alloy.createCollection('books');
    books.fetch({success: function(){  
        var authors  = Alloy.createCollection('authors');
        authors.fetch({success: function(){ 
            callback();
        },});
    },});
};

PROBLEM : above function returns callback 2 times. i just need to RETURN ONCE after both fetch are done.

(return true also returns twice...) is there any way that after both fetch are done, function returns callback only once?