form.haml

-# Note that in this scenario, there is only one comment.

= simple_form_for @post do |post_form|

  ...

  = post_form.simple_fields_for :comments do |comment_form|
    comment_form.input :name
    comment_form.input :email

  ...

  -# This should use the same model instance as the fields_for above, but doesn't.
  = post_form.simple_fields_for :comments do |comment_form|
    comment_form.input :blurb

models.rb

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

result.html

<form method="post" id="edit_post" class="simple_form post" action="/update_post" accept-charset="UTF-8">

  ...

  <div class="input string required">
    <label for="post_comments_attributes_0_name" class="string required">
      <abbr title="required">*</abbr> Name:
    </label>
    <input type="text" value="" size="50" required="required" name="post[comments_attributes][0][name]" maxlength="255" id="post_comments_attributes_0_name" class="string required">
  </div>
  <div class="input string required">
    <label for="post_comments_attributes_0_email" class="string required">
      <abbr title="required">*</abbr> E-Mail:
    </label>
    <input type="text" value="" size="50" required="required" name="post[comments_attributes][0][email]" maxlength="255" id="post_comments_attributes_0_email" class="string required">
  </div>

  ...

  <!-- This should use post_comments_attributes_0_blurb, not post_comments_attributes_1_blurb. -->
  <div class="input string required">
    <label for="post_comments_attributes_1_blurb" class="string required">
      <abbr title="required">*</abbr> Blurb:
    </label>
    <input type="text" value="" size="50" required="required" name="post[comments_attributes][1][blurb]" maxlength="255" id="post_comments_attributes_1_blurb" class="string required">
  </div>

  ...

</form>