Hi,
I'm not following you. You want the same address to always be the same for the billing and the shipping ?
In any cases, if you need to modify the behavior of the address system, you'll have to at least edit some of the address view files.
There are the address and address_select files of the checkout view and the show and edit file of the address view.
The cod is quite complex as it uses a combination of PHP, javascript, ajax requests and can handle different modes of functioning based on the "address selector" setting of the hikashop configuration.
If you need customization and are not a developer, you can contact our partners :
www.hikashop.com/home/our-partners.html