Have a read of the second tutorial - this covers adding extra fields to pages. Basically to add on to what aaron said, theres 3 steps
1 - Add the field to the Database
Open up mysite/code/Page.php and in the static $db = array() add a field with the type HTML Text. So it would look something like below. After you add this run yoursite.com/db/build?flush=1 to rebuild the database.
static $db = array(
'SecondContent' => 'HTMLText'
// insert extra fields here
);
2. Add the field to the CMS so you can write content! - Just as per the last post
// Add this method under your static $db array bit.
function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab('Root.Content.Main', new HTMLEditorField('SecondContent', 'Second Content'));
return $fields;
}
3. And last step is to add the variable $SecondContent to wherever you want to render the content in the template!