How to change email notification fuction in "Designer"?

Hi experts,

I want to implement an email notification function in IAM.

Here is the situation:
when somebody makes a request, both A and B will receive that request. And the only condition to approve this request is both A and B agree to the request. But when approver A rejects that request, approver B can't receive any email notification.

So, I want to change some configurations in "Designer" to make sure B will receive an email notification when A rejects that request. But I can't find any manual to do that. can any experts give me some instructions or demos? much appreciated!!!