Sessions with multiple domains going to one site

Please forgive me as I'm very new to Code Igniter. I had this project dropped in my lap and am trying to understand someone else's code.

I made a separate website for a client with the same backend as the original but different front end and a different domain name that hits the same server/db as the original.

So two domain names hit the same website, but with different front ends.

I don't know if this was an existing bug, or if the bug was caused by creating two sites, but the problem is that there is a cart on the site who's data is stored in a ci_session record. During checkout, it leaves the site to go to a third party site to pay, and then redirects back to their site. When it redirects back to their site, the cart is empty because it looks like a new session has been created with no cart data instead of going back to the original session and allowing our code to clear the cart properly.

I don't know if this is a timeout thing or something funky because of having two sites.  I also don't know if it's only my sessions that are being recreated because I'm in the US hitting a UK server (6 hours ahead), and that someone who would buy something in the UK wouldn't have a new session generated. US and UK are both experiencing the problem, but hard to know because of this whether the sessions do in fact play a role.

Any help would be greatly appreciated.