Proper approach to two different address data sets on the system
I'd just use Commerce Addressbook, it would allow you to have a set of addresses in your user account, from project page:


  Commerce Addressbook is a module that allows authenticated customers
  to reuse previously entered addresses during checkout. They can manage
  all entered addresses in their user panel (user/%user/addressbook).


Seems exactly what you're looking for and is way less complicated that dealing with the addresses and defaults yourself.