Validation rules in Laravel for joining two columns

Is there a way to build a validation in Laravel which checks if two columns together are unique in a table?

So for example if there is [(A, B)] in my table, then the input (A, A) or (B, B) should pass the validation but (A, B) again have to fail.

For example like this:

DB::table('mytable')
            ->where([
                ['col1', $item->col1],
                ['col2', $item->col2]
            ])
            ->exists();

But I want to do it within a validation, so something simular to this:

public function validate() {
        return Validator::make($this->attributes, [
            'col1|col2' => 'unique:mytable',
            //....
        ])->errors();
    }

via Chebli Mohamed