Allow multiple email to receive the access request.
When setting up Access request in a site collection, there is only one email address allowed. In 2010 we could add a semi-colon separated list of email.
it is all explained at https://jmitnotes.wordpress.com/2015/06/05/sharepoint-2013-access-request-email-must-be-unique/
Janko Schoettl commented
This is already possible if you simply use a comma istead of a semi-colon. There was a change in the source code from sharepoint between 2010-2013
I would prefer that the emails of the site owners (Full control) were set by default in Site settings > Site permissions > Access Request Settings. It would be automatically updated when the owners change. And why not, it could be overwritten by others emails if they were entered manually.