Complete form on site A and login on site B

I have a login form on my main website (Site A) that simply collects a username and password.

<form class="" method="post" action="http://ift.tt/1z7msGW">
        <h2 class="text-left">Login</h2>
        <hr/>
        <div class="form-group">
            <input type="text" name="email" class="form-control" placeholder="Username/E-mail Address" required/>
        </div>
        <div class="form-group">
            <input type="password" name="pass" class="form-control" placeholder="Password" required/>
        </div>
        <hr/>
        <a href="" class="btn btn-info btn-tiny pull-left">Editor Login</a>
        <input type="submit" class="btn btn-primary btn-tiny pull-right" value="Login"/>
    </form>

This form submits to my accounting program (Site B). The script looks like this:

    public function login()
{
    if($this->input->post())
    {
        $email = $this->input->post('email');
        $pass  = $this->input->post('pass');

        if($this->aauth->login($email, $pass))
        {
            redirect('/panda');
        }
        else
        {
            $this->error('/login');
        }
    }
    else
    {
        $this->theme->view('auth/login_v');
    }
}

When this happens, it looks like the login isn't getting registered because it redirects to the login page of site b and tells me I'm not authorized.

So how would I complete a form on one site and then send the form data to another and register the login session?