How to merge two UL with JQuery so that the values alternate?

I'm trying to merge two UL with JQuery so that the values alternate. How can I merge the following :

  <ul id="1">
    <li>1</li>
    <li>2</li>
    <li>3</li>
  </ul>

  <ul id="2">
    <li>a</li>
    <li>b</li>
    <li>c</li>
  </ul>

To something like this :

  <ul id="1">
    <li>1</li>
    <li>a</li>
    <li>2</li>
    <li>b</li>
    <li>3</li>
    <li>c</li>
  </ul>

With JQuery ?

Thanks.