How is 'Global Cart' supposed to work?

Hi there,

I am trying to figure out how to set up the global cart..

I have a multisite with global cart. I added one product on the main-store, and one product to the sub-store. First I add the main-store product to the cart - then the other, and goes to checkout from there (sub-store). In the sub-store checkout I have ability to add coupons from sub-store to the basket.. Coupons from main-store is not valid..
If I then go back to main-store and enter the checkout, the coupons from sub-store is gone and I am able to add coupons from main-store but not from the sub-store..

hmmm...

I have not tested to actually do the payment in both these cases, but I wonder..

Anyone?