and using the first website "only" for:
- manage users account (create, update, delete ... using API)
- display products (using API)
- manage products (create, update, delete ... using API)
- manage the users's cart (update ... using API)
and using the second website "only" for pay (checkout).
There are many plan: https://www.shopify.com/pricing
Can I use the Shopify API with the Basic Plan?
- yes, you can use both of your websites. but to transfer data from shopify to your admin site, you will need to have your custom app built. I suggest to only use shopify site which provides vast features to manage orders, customers, products etc. in short, you will not need your first website when you are using shopify.
- yes, you can access shopify API using basic shopify plan.