I think if you are coming from Rails and Ruby to Phoenix, you should learn elixir first. Or else, like a lot of people, you may think that it's similar to Rails. Phoenix may look similar to rails, but there is a huge difference on how they work. Also atleast from my point of view, you should know about OTP too. Your application is an elixir app, with phoenix which provides just the web interface to your business logic.

I learned Ruby and Rails at the same time just like you and many others. I found that people I got to work with who had learned Ruby first and then Rails had a clearer understanding of how things actually work. Rails was doing so much magic that some people never learned how things work without Rails. Autoloading is the strongest example that comes to mind.

You can learn both, but learning the basics of elixir will help. Phoenix is not your app, don't treat it like that.