If it’s really just two params that shall be the same (like password and it’s repetition), one might consider a plain pattern match and a guard that assures that two fields are the same and not nil.

def some_action(conn, %{"field_1" => value, "field_2" => value}) 
when not is_nil(value) do
  # do something with value 
end
def some_action(conn, _params) do
  # render "field_1 must be equal to field_"" 
end

For JSON payloads I’d use a JSON schema to validate against.

A contrived example

# file: lib/form_demo_web/controllers/page_controller.ex
#
defmodule FormDemoWeb.PageController do
  use FormDemoWeb, :controller

  # Store user form data under conn.params["user"]

  def index(conn, params) do
    # Data to preload form with
    user = %{"name" => "Bruce", "username" => "redrapids"}

    conn
    |> update_user(user)   # add "user" to conn
    |> render("index.html")
  end

  def update(conn, %{"user" => user}) do
    case validate_user(user) do
      {:ok, user} ->
        conn
        |> update_user(user)
        |> put_flash(:info, "Updated to #{user["name"]}, #{user["username"]}!")
        |> render("index.html")

      {:error, errors} ->
        render(conn, "index.html", errors: errors)
    end
  end

  # Make data available under conn.params["user"]
  # for Phoenix.HTML.Form.form_for/4
  #
  # Plug.Conn implements Phoenix.HTML.FormData used by form_for/4
  #
  defp update_user(conn, user),
    do: Map.update!(conn, :params, &Map.put(&1, "user", user))

  defp validate_user(user) do
    errors =
      []
      |> check_value(user, :name)
      |> check_value(user, :username)

    case errors do
      [] ->
        {:ok, user}

      _ ->
        {:error, errors}
    end
  end

  def check_value(errors, user, key) do
    cond do
      Map.get(user, Atom.to_string(key), "") == "" ->
        # format expected by FormDemoWeb.ErrorHelpers.error_tag/2
        [{key, {"can't be blank", []}} | errors]

      true ->
        errors
    end
  end
end

# file: lib/form_demo_web/router.ex
#
defmodule FormDemoWeb.Router do
  use FormDemoWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", FormDemoWeb do
    pipe_through :browser

    get "/", PageController, :index
    post "/", PageController, :update # <- Added to validate form data
  end
end

# file: lib/form_demo_web/views/page_view.ex
#
defmodule FormDemoWeb.PageView do
  use FormDemoWeb, :view

  def get_errors(%Plug.Conn{assigns: %{errors: errors}}) when is_list(errors),
    do: errors

  # FormDemoWeb.ErrorHelpers.error_tag/2 needs a list
  def get_errors(_),
    do: []

  def errors?(%Phoenix.HTML.Form{errors: [_ | _]}),
    # non-empty list of errors
    do: true

  def errors?(_),
    do: false
end

<!-- file: lib/form_demo_web/templates/page/index.html.eex -->
<!--
     Phoenix.HTML.Form.form_for/4
     Phoenix.HTML.Form.text_input/3
     Phoenix.HTML.Form.submit/1

     @conn --- Plug.Conn implements Phoenix.HTML.FormData protocol
     Routes.page_path(@conn, :update) --- generates the URL to PageController.update/2
     [as: :user] --- form data is stored under conn.params["user"]
     [errors: get_errors(@conn)] --- extracts the errors for the conn to include it in the Phoenix.HTML.Form struct
     errors?/1, get_errors/1 --- defined under FormDemoWeb.PageView
     error_tag/2 --- from generated module FormDemoWeb.ErrorHelpers
-->
<h1>Current User</h1>
<%= form_for @conn, Routes.page_path(@conn, :update), [as: :user, method: "post", errors: get_errors(@conn)], fn form -> %>
  <%= if errors?(form) do %>
    <div class="alert alert-danger">
      <p>Oops, something went wrong! Please check the errors below.</p>
    </div>
  <% end %>
  <div>
    <%= text_input form, :name, placeholder: "Name" %>
    <%= error_tag form, :name %>
  </div>
  <div>
    <%= text_input form, :username, placeholder: "Username" %>
    <%= error_tag form, :username %>
  </div>
  <%= submit "Update User" %>
<% end %>

<!DOCTYPE html>
<html lang="en">
  <!-- file: lib/form_demo_web/templates/layout/app.html.eex -->
  <head>
    <meta charset="utf-8"/>
    <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
    <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
    <title>FormDemo · Phoenix Framework</title>
    <link rel="stylesheet" href="<%= Routes.static_path(@conn, "/css/app.css") %>"/>
  </head>
  <body>
    <main role="main" class="container">
      <p class="alert alert-info" role="alert"><%= get_flash(@conn, :info) %></p>
      <p class="alert alert-danger" role="alert"><%= get_flash(@conn, :error) %></p>
      <%= render @view_module, @view_template, assigns %>
    </main>
    <script type="text/javascript" src="<%= Routes.static_path(@conn, "/js/app.js") %>"></script>
  </body>
</html>