How can i assign and simpleXMl object to another simple xml Object.

Question

How can i assign and simpleXMl object to another simple xml Object.

For example:

$xml = simplexml_load_string("<tests><test1>test 1 </test1><test2>test 2 </test2></tests>");
    $xml1 = simplexml_load_string("<test1><name>test 1</name><cat>xyz</cat></test1>");

    $xml->test1 = $xml1;
print_r($xml);

it returns me this

SimpleXMLElement Object ( [test1] => SimpleXMLElement Object ( [0] => ) [test2] => test 2 ) 

Really need some help