Include Search results from multiple sites

I have two sites and I would like all the content on both sites to show up in the search results when someone searches at either site. My search function is as follows:

<form method="get" id="searchform" action="<?php bloginfo('url'); ?>/">
  <div>
    <input name="s" type="text" id="s" value="<?php the_search_query(); ?>" size="15" />
    <input type="submit" id="searchsubmit" value="Search Site" />
</div>
</form>

Any idea on how to add another site?

My two sites are Prom Mafia and Glam Mafia.

Thanks!