Hello,
starting in Alfresco 4 you can use Share / Surf Extension Modules to modify the login page design depending on some information in the request. You need to be able to uniquely identify the customer via the HTTP request though or you won't be able to target them correctly. In Alfresco 3.x, the only way I see would be to provide two distinct login pages and route the customers to the correct ones outside of Share (i.e. in an Apache proxy, again based on HTTP request information).
Regards
Axel