Okay, it's in blogs/htsrv/message_send.php

I have a feeling I should edit this line to take a string:

param( 'recipient_id', 'integer', '' );

and then parse it for the +. Is this what I should do?

If you want a second email to be sent to a fixed recipient (eg webmaster) then you can just append his emailadress to the first. Example:

$recipient_adress = $recipient_name.'<'.$Comment->get_author_email().'>;
$recipient_adress = $recipientadress.', <youremailadress>';

If on the other hand nine is a neighbor of eight and they both want to receive the email, you'll need to go through a loop because you need to do $Comment->get_author_email() twice.
As far as I can see the recipient_id is a number, that means you can just do:

$recipient_id_2 = $recipient_id +1;

now $recipient_id_2 holds the value 9 and $recipient_id holds the value 8.
This is all very static and I don't see a purpose for it. Perhaps a clarification can trigger some more ideas.

Good luck