User Login and Signup for App with Multiple, but Separate, Roles

I'm building an app that will have two different types of users, let's say "Shoppers" and "Agents" for example.

I am trying to determine whether I should split the login / signup to different areas entirely, or have 1 login for bother user types. I will be doing Facebook and Google logins, so I am thinking that I need to somehow prompt the user which type they are before they do any other action (e.g. click the log in or sign up with facebook button).

What I am ultimately trying to do is keep the process simple and easy, without the user signing up as the wrong type from the beginning.

Any ideas or examples of sites or apps that are currently doing this or similar process?