multiplex_tweet.user.js

// ==UserScript==
// @name           Multiplex Tweet
// @namespace      http://aspietribe.com/
// @include        http://twitter.com/
// ==/UserScript==

var times = 5;

document.getElementById('tweeting_button').parentNode.addEventListener('click', function(e) {
  post();
  e.preventDefault();
  e.stopPropagation();
}, true);

function post() {
  var tweet = document.getElementById('status').value;
  var form = document.getElementById('status_update_form');
  var evt = document.createEvent('HTMLEvents');
  evt.initEvent('submit', true, true);
  evt.element = function (){ return evt.target; };
  for(var i = 0; i < times; i++) {
    form.dispatchEvent(evt);
  }
}