[Ask] How to pass cart content to other site in different domain?

[eluser]Unknown[/eluser]
I have two identical CI website, say site A and site B. For some reasons, I need to pass the cart content from site A to site B. I found that the cart will be empty when the site A redirected to site B. I believe that it has something to do with the session, but I don't have any idea how to fix it. Is it possible to use original cart library provided by CI? Can anyone give me a clue how to do it? Thank you.