This kind of scenario comes up when we want same set of fields to be repeated in core modules.
For eg., if I want order status page to be displayed in Sales and Purchase Order both. So, I define a order_status object which holds the common fields. Then I will inherit sale_order and purchase_order objects and also inherit order_status object.
Now, the next question is to how to assign more than one object to _inherit attribute. The trick is to define multiple class in list as follows-