ZenCart - Allow for BOTH Default Billing and Shipping Address per Customer

We're running ZenCart version 1.5.6c It does have a good bit of customization done to it, and 3rd party modules installed. For our ZenCart checkout, we are using the One-Page Checkout module ([login to view URL]) Currently, a customer can make a particular address as "Primary," however, it will make it be primary for BOTH Billing & Shipping. We want the ability for our customers to have the Billing address different from the ship to address, but also to be able to specify a Primary Shipping Address that is used by default if multiple addresses exist in the address book. Please state your experience with ZenCart - How many years and since which version have you been involved?, some sites you've developed, How well do you know JQuery, CSS, PHP, MySQL? What are the top 5 modules (3rd party) that you install? Thanks!