Multi-store shared cart

http://www.diapers.com/
How was this done ? using multible stores / brands

I have been so unfortunate to get this done ... www.bilstereoklub.dk
But not aware that i now only have 1 cart pr store..

And not 1 cart for all stores..
Plus the same client account for all stores.

If anyone can help coding this so it works...
and works safely..
Then i would like to talk business.