MultiStore - Same cart

Hello,
Anyone could help me with the following:

I have some OC webshop operating as a multistore. However I wish that custmer could
- access all stores by loginning in any of them
- has one cart gathering all products bought in the different stores
- pay for all items in one transaction

Please let me know if you could help me out (Also interested in business solution, just send me an offer)
Thanks