Update 2 tables with one form.

I have a form:

<% form_tag :action => 'create' do %>
  <%= hidden_field_tag 'plan', params[:plan] %>

  <fieldset>
    <% fields_for :account do |f| %>

    <%= f.text_field :name %>

      <%= f.text_field :domain %>.<%= AppConfig['base_domain'] %>
    <% end %>
  </fieldset>

  <fieldset>
    <% fields_for :user do |f| %>
    <%= f.text_field :login %><br/>
      <%= f.text_field :eservice %> PPL Eservice Login (If Known)<br/

    <%= f.text_field :first_name %><br/>
    <%= f.text_field :last_name %><br/>
      <%= f.text_field :email %><br/>
      <%= f.text_field :contact_number %><br/>
      <label for="user_password">Password</label>
      <%= f.password_field :password %><br/>
      <label for="user_password_confirmation">Password Confirmation</

      <%= f.password_field :password_confirmation %>
    <% end %>
  </fieldset>

I would like to only have the second part of the form and have the
fields first_name last_name update the account table name and have
login update the account table domain. Do you know how to do this?

Thank you,

Sean McGilvray