gistfile1.js

var CallbackObject = {
  task_a_done = false,
  task_b_done = false,  
  trigger = function(which_task) {
    if(which_task == "A") { this.task_a_done = true; }
    if(which_task == "B") { this.task_a_done = true; }

    if(this.task_a_done && this.task_b_done) {
       trigger_task_c();
    }
  }
}

var ajax_task_a = Ajax.new({ onComplete: function() { CallbackObject.trigger("A"); }};
var ajax_task_b = Ajax.new({ onComplete: function() { CallbackObject.trigger("B"); }};