Time with Erlang is not necessary beforehand but do go in with the knowledge that there is a lot of interop given you can call Erlang code in much the same way as Elixir code.

Elixir made a decision early on to not deliberately wrap Erlang libraries that were perfectly sane as they stood. For example, to get a simple queue construct in Elixir you just call the Erlang queue module.

Once you're up and running with Elixir then there is nothing stopping you delving deeper into Erlang and OTP.