You can technically pass multiple feed URLs to fetch_feed() as an array, and it’ll grab them all at once. But the return is a total mess, and I couldn’t personally figure out how to parse it.
If nobody else knows how to do this with fetch_feed(), I can offer a solution using the SimplePie class (which fetch_feed actually uses anyway). The SimplePie class has some helper methods for parsing the return, and they make things pretty easy.
Try something like:
// Contains the SimplePie class
require_once (ABSPATH . WPINC . '/class-feed.php');
// New class instance
$feed = new SimplePie();
// You feed URLs
$feed->set_feed_url(array('http://rss.cnn.com/rss/cnn_topstories.rss', 'http://cuteoverload.com/feed/'));
// Do it!
$feed->init();
$feed->handle_content_type();
// Loop the results
foreach($feed->get_items() as $item) {
echo $item->get_title();
echo '<hr/>';
}
Additional SimplePie methods include get_permalink() and get_description().
The only downside to this approach is that is SimplePie is ever phased out of WordPress in favor of another class, this’ll break.
UPDATE
As @Rarst pointed out in the comments, you don’t need to access SimplePie directly. You can use its methods on the object that fetch_feed() returns. So the answer is much simpler than I thought:
$feed = fetch_feed(array('http://rss.cnn.com/rss/cnn_topstories.rss', 'http://cuteoverload.com/feed/'));
// Loop the results
foreach($feed->get_items() as $item) {
echo $item->get_title();
echo '<hr/>';
}