There is a long way here
Create a php file(share.php) as following in both domains, data is shared by this method
<?php session_start();
echo json_encode($_SESSION);
?>
Now create another file, better to use as include, on another domain for receiving session data
<?php session_start();
$abc = file_get_contents('http://domain1.com/share.php');
$obj = json_decode($abc);
print_r($obj);
?>
print_r will show you whole session data of another server in this way. Use combination of objects and arrays to access all elements.
If you need to overwrite this server's session use this code instead of print_r
<?php
foreach($obj as $key => $value){
$_SESSION[$key] = $value;
}
?>