Right now if I edit only default billing address then automatically update default shipping address. Can anyone have any code idea to customize this?
Default billing and default shipping are references to an address.
If you have the same address marked as billing and shipping then they both reference the same address object. So it is normal that if you edit one, you edit the other because basically they are the same address.
If you want separate addresses, create 2 of them and mark one as billing and one as shipping.