In Yii2 how to validate multiple attributes together. Using AJAX

I have tried following this https://github.com/samdark/yii2-cookbook/blob/master/book/forms-validator-multiple-attributes.md Unfortunately, it already has an issue raised that it no longer works with updates to Yii2 and I can't find any way to make it work.

I have two attributes which must be unique from each other. (Both are an array of strings.)

My validation logic works, but I can't find a way to display errors (or remove errors) on both attributes at the same time.

I have tried making a custom validator class, and also an inline validator, with the same problem:

When entering data in the form, only the attribute being edited will have its error message updated.

Below is the simplest version of my code that doesn't work, I want to display an error on both attributes after editing either one of them.

public function rules()
    {
        return [
        [['attribute1', 'attribute2'], 'customValidator'],
        ];
    }

// declare validator
public function customValidator($attribute, $params)
    {
        $this->addError('attribute1', 'error');
        $this->addError('attribute2', 'error');
    }