A merchant asks you to extend customer functionality to allow customer accounts to be associated...

By adding the attribute like customer_address_billing2 and customizing both My Account and Checkout functionality to use that new attribute

By changing the System Configuration setting: Customer>Allow multiple billing addresses to Yes

By altering the customer_entity table, adding the field billing_address2, and customizing both My Account and Checkout functionality to use that new field

This is out-of-the box functionality