A single mix.exs file builds a single elixir application. If you want to have multiple applications within a single project you can use a mix umbrella project. Can you show your mix.exs file?

When I do development like that I usually just run each of the apps tests individually. You could also write a small bash script that would run each test suite (or a makefile)

Having two distinct applications a la PragDave has its advantages, but for the moment, I think I like the idea of the umbrella app. This is my current mix.exs

defmodule Estimator.MixProject do
  use Mix.Project

  def project do
    [
      app: :estimator,
      version: "0.1.0",
      elixir: "~> 1.8",
      start_permanent: Mix.env() == :prod,
      deps: deps()
    ]
  end

  # Run "mix help compile.app" to learn about applications.
  def application do
    [
      extra_applications: [:logger],
      mod: {Estimator.Application, []}
    ]
  end

  # Run "mix help deps" to learn about dependencies.
  defp deps do
    [
      {:plug_cowboy, "~> 2.0"},
      {:jason, "~> 1.1"},
      {:csv, "~> 2.3"},
      {:ex_doc, "~> 0.19", only: :dev, runtime: false}
    ]
  end
end

Thank you