How to fill_in field in one of two forms in a HTML file using capybara?

I have two forms (a sign-up and sign-in) in one html file which I am testing using Capybara's fill_in syntax. However both have a username field and both have a password field and I'm getting Ambiguous match, found 2 elements matching field :username . How can I, in my spec helper methods ensure I'm filling in the field of the correct form. I would imagine I need to id= each form but I can't find anywhere that has a capybara syntax for that.

My html

<form id="signUpForm"action='/user' method="post">
      <!-- more fields -->
      <input class="entryField" type="text" name="username" required>
      <input class="entryField" type="password" name="password" required>
      <!-- more fields -->
      <input type="submit" value="Sign up">
    </form>

    <form id="signInForm" action="/session" method="post">
      <input class="entryField" type="text" name="username" placeholder="Username">
      <input class="entryField" type="password" name="password" placeholder="Password">
      <input type="submit" value="Sign in">
    </form>

And my helper method

    def sign_in user
      visit '/'
      fill_in :username, with: user.username
      fill_in :password, with: user.password
      click_button 'Sign in'
    end