To send data to different parts of a page the use of templates is advised, at least by myself.
For instance, a template page file look like this
$this->load->view('tops/home1'); //header
$this->load->view($content);//main content(your login area)
$this->load->view('bottoms/main_home'); //footer
From the controller
$data['content'] = 'home/home_content'; //this is the **main** part of your page
$this->load->view('template1', $data);
To include your login script, simply put the login file in the root(in a directory made by you) and include it as a normal php include wherever you want it to be. As Aren noted, the use of Iframes is ancient and should be avoided