How to pass session cross different domains?

Now i am working on an cart application, when i add an item to cart(in
the
domain of http://www.example1.com), and then checkout(in the domain of

https://www.example2.com), 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
into

the checkout url, as
http://www.example2.com/checkout?host=www.example1.com&session_id=xxxxxxxxxxxxxxxxxxxxx).Whencheckout
it would redirect to

http://www.example2.com/checkout?host=www.example1.com&session_id=xxxxxxxxxxxxxxxxxxxxx.
Now the problem i could not get the data from the session.

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