How to pass session cross different domains?

Now i am working on an cart application, when i add an item to cart(in
domain of, and then checkout(in the domain of, i still want to get the item from the cart. I
used the plugin of ssl requirement, and i append the host and session_id

the checkout url, as
it would redirect to
Now the problem i could not get the data from the session.

Further more, i used memcached as session store. Is there anyone met
problem ever?