It would be very helpful if the customer data (phone#, address, email..) was shared between all stores. It would be much easier to spot the duplicates. Sometimes we have a customer that has only phone# in a store and complete data in the other, it makes no sense having to copy and paste that.
If the customer was a cathegory and the store a sub-cathegory all customers would be a shared database for all stores in the same group.