How to make an online code editor which two people can change the code at the same time?

I want to make an online code editor which two people can work on the same code(deleting, adding, or selecting code etc.) at the same time. I want to make pair coding much more enjoyable and effective this way. Instead of telling people what is in your mind you can show it in the editor which is much more understandable.

I am not very experienced in web development thus I am not very sure how to start doing it. I need framework and resource recommendations. Which framework would be suitable for this project? Any concept I need to learn before designing it? Any similar projects that can make me understand this project better? What should I learn before doing this project? Everything that can give me a head start is welcome.