One way I can think of is like this:
make a hash with visitor IP and browser and send it along with encoded (base64?) $_SESSION
data to second server like:
file_get_contents('http://dev.xyz.com/gateway.php?session=$IPBrowser&session=$session')
on the second server save that to a temporary table?
/file
and redirect user to second domain.
then you check his info based on IP and Browser and apply SESSION data.
this a VERY vulnerable solution, but it's a solution. I posted this thinking you are using for dev
purposes.
And no, I will not make the source code.