Why Session_id Column from Customer_visitor Table Deprecated in Magento 2.4

If you feel like hitting it with very large hammer you could also try to purge content of var/session catalog.

You say that the customer fields part you know.
This is a good start.
You should create an other customer attribute called 'secondary_email'. Make it mandatory if needed.

After that you have to change all the methods that send e-mails to customers and make them send and e-mail to the second address also.
Let's take for example Mage_Sales_Model_Order::sendNewOrderEmail().

There is this line that sets the e-mail address and name for the customer to the e-mail.

$emailInfo->addTo($this->getCustomerEmail(), $customerName);

You need after that this:

if ($this->getCustomer() && $this->getCustomer()->getSecondaryEmail()) {
    $emailInfo->addTo($this->getCustomer()->getSecondaryEmail(), $customerName);
}