A way to do this is to find both at the same time in the same loop...
This is a working code:
$url = "http://sportzcosmos.com/2014/03/29/european-football-leagues-weekend-predictions/";
//Create a DOM object
$html = new simple_html_dom();
// Load HTML from a url
$html->load_file($url);
$articlecontent = array();
foreach( $html->find('article.post div.entry-content p, article.post div.entry-content h2') as $article ){
$articlecontent[] = $article->plaintext;
}
print_r($articlecontent);
Output