How to perform the upload with multiple files

I use the following code in my form .

Code:

<input type="file" name="bbs_images[]">
<input type="file" name="bbs_images[]">

I tried

PHP Code:

$this->upload->do_upload('bbs_images'); 


and got 

Code:

Message: is_uploaded_file() expects parameter 1 to be string, array given

I also tried

PHP Code:

$this->upload->do_upload('bbs_images[0]'); 


 but it also returned false because do_upload would finally try to access $_FILES['bbs_images'][0] and finally got NULL.

So, How could I use Upload::do_upload since the structure of $_FILES['bbs_images'] is as follows ?

PHP Code:

array (size=5)
  'name' => 
    array (size=2)
      0 => string 'XXX.xls'
      1 => string 'XXX.sql'
  'type' => 
    array (size=2)
      0 => string 'XXX'
      1 => string 'XXX'
  'tmp_name' => 
    array (size=2)
      0 => string 'XXX'
      1 => string 'XXX'
  'error' => 
    array (size=2)
      0 => int 0
      1 => int 0
  'size' => 
    array (size=2)
      0 => int XX
      1 => int XXX