To query by multiple conditions pass them in an array like this:
$this->db->select('*');
$this->db->from('table');
$this->db->where(array('condition'=>$var_1, 'condition_2'=>$var_2, 'condition_3'=> $var_3)):
$result = $this->db->get()->result_array(); //returns result as array
However it depends on your specific query, as certain types of queries/conditions don't work in this type of format.
EDIT: this is what you are looking for though:
$this->db->select('*');
$this->db->from('table');
$this->db->where('condition_1'=>$var_1);
$this->db->where("(condition_2=1 OR condition_2=2)", NULL, FALSE);
$query = $this->db->get()->result_array();