CodeIgniter：在另一个Controller中加载多个控制器

I'm fairly new to CodeIgniter and trying to do the following:

I've got a model App, Attribute and Sound.

In the create method of the App-Controller, I'm trying to save Attributes and Sounds. I'm using the following (basically working) approach:

$this->load->library('../controllers/attributes');
$this->attributes->create('value1', 'value2');

This works perfectly, the attributes controller is found in $this->attributes.

Now I'm trying to do the same for the sounds:

$this->load->library('../controllers/sounds');
$this->sounds->create('value3', 'value4');

This doesn't work, because the sounds controller is saved in $this->attributes->sounds instead of $this->sounds.

Edit:

$this->load->library(array('../controllers/sounds', '../controllers/attributes'));

Results in the same problem, it'll turn into $this->sounds and $this->sounds->attributes.

Where is my mistake?