Change Login Url For Cwebuser Multi Object ?

i have configured two CWebUser object for backend/frontend in my web applications. But when I implement access control list on backend background, it allway used redirect login url default of user component. How to change login url for backend background

my config like

'user'=>array(

    'class'=>'CWebUser',

    'stateKeyPrefix'=>'user_',

    'loginUrl'=>array("site/login"),

),

'backendUser'=>array(

    'class'=>'CWebUser',

    'stateKeyPrefix'=>'admin_',

    'loginUrl'=>array("backend/login"),

),

How did you configure redirect to your login in the first place? i have a similar challenge