Ruby on rails sign up and sign in for two kinds of users

I am trying to add two kinds of users for login, there is already internal user login where user uses their own internal email, so there is no sign up step(done before go to the app). Now I want external users to use the app, so a sign up is needed before they login, I don't want to change the current app a lot so I want to do this without devise. I am planning to create a new model of external user, and a controller for it as well, when external users open the app, they will first see the internal login page, and on this page, I add a link for them to go to the register page, I am wondering if this will work, then it means there will be two controllers users_controller and external_users controller for the login/sign-up of one app. Can I use the same session_controller for both internal_users and external_users? Or if there is any other approach that is better? Thank you.