class DOM_Object_1 {

    private $DOM;
    public $DOM_Object_2;

    public function __construct( \DOMDocument $SLDDOM ) {
        $this->DOM = $SLDDOM;
    }

    public function __clone() {
        $this->DOM_Object_2 = clone $this->DOM_Object_2;
    }

    public function getDOMNode() {

        $fragment = $this->DOM->createDocumentFragment();
        $node = $fragment->appendChild( $this->DOM->createElement( "Knoten_1" ) );
        $node->appendChild( ( $this->DOM->createTextNode( "Textinhalt 1" )) ) ;

        if( isset( $this->DOM_Object_2 ) ) {
            $font = $node->appendChild( $this->DOM->createElement( "DOM_Object_2" ) );
            $font->appendChild( $this->DOM_Object_2 );
        }

        return $fragment;
    }
}

class DOM_Object_2
{
    private $DOM;

    public function __construct(\DOMDocument $SLDDOM)
    {
        $this->DOM = $SLDDOM;
    }

    public function getDOMNode()
    {
        $fragment = $this->DOM->createDocumentFragment();
        $node = $fragment->appendChild($this->DOM->createElement( "Knoten_2" ) );
        $node->appendChild(($this->DOM->createTextNode( "Textinhalt 2" ) ) );
        return $fragment;
    }
}

$DOM = new \DOMDocument;
$DOM->preserveWhiteSpace = false;
$DOM->formatOutput = true;

$root = $DOM->appendChild( $DOM->createElement( 'root' ) );

$DOM_Object_1 = new DOM_Object_1( $DOM );
$DOM_Object_2 = new DOM_Object_2( $DOM );

$DOM_Object_1->DOM_Object_2 = $DOM_Object_2->getDOMNode();

$root->appendChild( $DOM_Object_1->getDOMNode() );

echo $DOM->saveXML();