I can only suggest you this:
Use a page with frames, having one of them to 0 width and height. In that, frame, open your page of the other server, and make the client start a session in there.
So when the customers clicks on the link "Go to ComicFrames.com and see your buying cart there", the main frame can refresh and show the contents of the other site, WITH session started. This would allow you to see both buying carts, but not to get them together. Now, about the cart, if you want to keep it together, i suggest you use a DB. Store the queries from the customer as soon as he/she clicks on them, and, later, when processing the cart, delete those that the customer don't want. Of course this database should be accessed by both servers. You could make this DB apart from the rest (or maybe keep it in tables on one of the sites)
For this session thing not be a problem, you should set your session lifetime to long time.
I know it's not the best solution, but at least it works (i tried t and yes, amazingly, it does)
Hope this helps
fLIPIS