I'd say jump into the non-OTP language features of Elixir first to get the basics of Elixir/Erlang's approach to FP. Maybe do some of your own process spawning and linking/monitoring. After that dig into "Learn You Some Erlang"[0] and/or "Erlang/OTP in Action" to learn what OTP is and how to use it.

I definitely recommend Elixir first, though. The development tooling are so good that they get out of the way while you learn the language whereas in Erlang you'll be immediately confronted by its lack/complexity of tooling.

0 - http://learnyousomeerlang.com

1 - http://www.manning.com/logan/