How do you set up CI2 to allow extending of multiple models?

I can only get it to extend one model (put in /application/core) named MY_Model (case sensitive).

To choose what model to extend I am doing; in the model..

require_once APPPATH.'core/MY_Another_model.php';
class Test_model extends MY_Another_model {
...
}

I can't find where in the core system code where it states only to allow models that are being extended to be called MY_Model.

Thank you for any and all help.