I was able to compare the dates of posts and news and add the newest to a new array. Then send the new array to the rss feed bundle.
$news = $em->getRepository('ACMENewsBundle:News')->getLatest();
$posts = $em->getRepository('ACMEPostsBundle:Posts')->getLatest();
$latest = [];
$latest_news = 0;
$latest_post = 0;
for ($i = 0; $i < 7; $i++) {
if ($news[$latest_news]->getUpdated() > $posts[$latest_article]->getUpdated()) {
$latest[$i] = $lessons[$latest_news];
$latest_news++;
} else {
$latest[$i] = $posts[$latest_post];
$latest_post++;
}
}
$feed = $this->get('eko_feed.feed.manager')->get('article');
$feed->addFromArray($latest);
return new Response($feed->render('rss'));