I just combined two or more fields into 1, and that worked!
So you have to write something like this:
add_filter( 'wpseo_pre_analysis_post_content', 'filter_yoasts_wpse_119879', 10, 2 );
function filter_yoasts_wpse_119879( $content, $post )
{
$field1 = get_field( 'text', $post->ID );
$field2 = get_field( 'more_text', $post->ID );
$fields = $field1 . ' ' . $field2;
return $content . $fields;
}