class First_model extends Model
{
function First_model()
{
parent::Model();
}

function First_Function()
{
echo 'This is parent Function';
}
}

class Second_model extends Model
{
function Second_model()
{
parent::Model();

//Load First Model
$ci=& get_instance();
$ci->load->model('First_model');
}

function Second_Function()
{
//Call First model Function
$ci->First_model->First_Function();

}
}