Hello,
You should not try to join them in these two datasource in BW. Best Practice is to load them in separate objects.
0customer_attr ---> in 0Customer
0cust_sales_attr --> in 0cust_sales
Keep both objects in your transactional data i.e. in cubes. All navigational attributes of these two infoobject will be available in the report. THis is Best practice and most of the companies implement master data in this way..
Regards
Saurabh