If you have more than one TypeKit that you need to select at runtime, this function is a way to do that. This uses d3 but is easily convertible to plain JS.

load_a_typekit.js

// Adds a script tag using the given TypeKit url. When the TypeKit script 
// arrives, it loads the TypeKit and calls the callback.

function loadATypeKit(typekitURL, done) {
  var head = d3.select('head');

  var typekitScript = head.append('script').attr({
    type: 'text/javascript',
    src: typekitURL
  });
  var typekitScriptEl = typekitScript.node();
  typekitScriptEl.onload = function loadTypeKit() {
    try {
      Typekit.load({
        active: done,
        inactive: done
      });
    } 
    catch (e) {
      debugger;
    }
  };
}

function loadARandomTypeKit() {
  if (Math.floor(Math.random() / 2) === 0) {
    loadATypeKit('//use.typekit.net/aaaaaaa.js', doneLoadingTypeKit);
  }
  else {
    loadATypeKit('//use.typekit.net/bbbbbbb.js', doneLoadingTypeKit);
  }
}

function doneLoadingTypeKit() {
  console.log('done!');
}