class A extends yiidbActiveRecord
{
    public $category;

    public function rules()
    {
        return [
            [['category'], 'safe'],
        ];
    }



}

{
    public $item;
    public $category;

    public function rules()
    {
        return [
            [['item'], 'safe'],
            [['item'], 'required', 'when' => function($model) {
                return $model->category == 'yes';
            }]
        ];
    }
}

$a = new A();
$b = new B();
if ($a->load(Yii::$app->request->post()) && $b->load(Yii::$app->request->post())) {
            $b->category= Yii::$app->request->post()['First']['category'];

            $isValid = $a->validate();
            $isValid = $b->validate() && $isValid;
            if ($isValid) {
                echo 'its valid';
            }


        }