We have mutiple domains, and in most cases users should only receive emails on one domain. But in some rare cases, we have a few users that should receive on two domains (but not all domains)

My case:
One user with first middle last wants to use the following address, first.last@domain.com.
He also wants to receive on first.last@domain.org.
Because he should not receive any email on other domains, I have checked "For incoming mail, recipients are known exlusively by this domain".

Is this possible? How do I go further?