1.5.1.1 Multistore - customer cannot buy at both stores?

Hi
I am using a clean install of 1.5.1.1 and set up two stores using multistore.

Problem occurs when customer makes first purchased at main store, then later tries to buy from other store - they cannot complete the purchase, getting errors during checkout - at step 5 in checkout (payment method) receiving multiple errors.

When I made the product they were trying to buy available on the main store to test the problem - they could purchase without issue.

I have searched the forums but have not found anyone reporting similar, so I am not sure what I did wrong, or what the issue is.

Error logs from the first time they tried are:
2011-11-14 8:32:31 - PHP Notice: Undefined index: shipping_address_id in /home/www/mainstore.com/catalog/controller/checkout/confirm.php on line 28
2011-11-14 8:32:31 - PHP Notice: Undefined index: shipping_address_id in /home/www/mainstore.com/catalog/controller/checkout/confirm.php on line 143

They then tried again and there were multiple errors - all in the admin/controller/sale/customer.php file.

2011-11-14 8:58:57 - PHP Notice: Undefined index: firstname in /home/www/mainstore.com/admin/controller/sale/customer.php on line 798
2011-11-14 8:58:57 - PHP Notice: Undefined index: lastname in /home/www/mainstore.com/admin/controller/sale/customer.php on line 806
2011-11-14 8:58:57 - PHP Notice: Undefined index: email in /home/www/mainstore.com/admin/controller/sale/customer.php on line 814
2011-11-14 8:58:57 - PHP Notice: Undefined index: telephone in /home/www/mainstore.com/admin/controller/sale/customer.php on line 822
2011-11-14 8:58:57 - PHP Notice: Undefined index: fax in /home/www/mainstore.com/admin/controller/sale/customer.php on line 830
2011-11-14 8:58:57 - PHP Notice: Undefined index: newsletter in /home/www/mainstore.com/admin/controller/sale/customer.php on line 838
2011-11-14 8:58:57 - PHP Notice: Undefined index: customer_group_id in /home/www/mainstore.com/admin/controller/sale/customer.php on line 850
2011-11-14 8:58:57 - PHP Notice: Undefined index: status in /home/www/mainstore.com/admin/controller/sale/customer.php on line 858

I do not know what this means as I am not a coder, can anyone help with this?

I expect the same purchase situation to be happening more often in the future, as more customers buy from the main shop first, then want to buy products which are only available on the second store once they are a customer.

Anyone come across this themselves - or can advise?

Many thanks for any guidance!

This is true. There is only one customer database, which means that a customer who registers at any one of your stores can shop at any one he chooses. However, there is also only one session manager, which means that shopping in one store must be completed before shopping at another store can begin.

It is just like being a member of a wholesale club. You become a member at one club location and you are permitted to shop at any location you please. Some products, like particular bakery items, vary from location to location. Even though you can shop in any store, you cannot take your half full shopping cart from Location A and go to Location B across town and expect to pay for your entire order when you finish at Location B.

Thanks mberlant for your very helpful and well explained post :-)

I did not know that is what the errors meant and having looked in 'abandoned orders' can see that on all but 2 occasions, the customer did have items from different shops in their basket.
Still not sure what was happening on the 2 times when only products from one shop were in basket, but your post gives us something to be aware of - many thanks again! :-)