MongoDB few collection in one Model

Hi, how i can use 2 collections in one Model. e.g.

class Admin extends Collection{
    public function getSource(){
        return "user1"
    }
    public static function findUser1(){
        $user1 = new Admin();
        $user1->setSource("user1");
        $cursor = $users1->find()
        foreach($cursor as $doc){
            echo $doc['login']
        }
    }
    public static function findUser2(){
        $user2 = new Admin();
        $user2->setSource("user2");
        $cursor = $users2->find()
        foreach($cursor as $doc){
            echo $doc['login']
        }
    }
}

But, will work only findUser1.

setSource work only with insert operation, where i mistake ?