<?php
$embed1 = get_post_meta(get_the_ID(), 'embed1', true);
$embed2 = get_post_meta(get_the_ID(), 'embed2', true);

if ( ! empty( $embed1 ) ) {
  echo '<iframe  scrolling="no" frameborder="0" width="100%" height="360" allowfullscreen="true" webkitallowfullscreen="true" mozallowfullscreen="true" src="' . $embed1 . '"></iframe>';
} elseif ( ! empty( $embed2 ) ) {
  echo '<iframe  scrolling="no" frameborder="0" width="100%" height="360" allowfullscreen="true" webkitallowfullscreen="true" mozallowfullscreen="true" src="' . $embed2 . '"></iframe>';
}
?>