Customer email addresses - saving multiple for Invoices or Sales Order confirmations

We would like to have multiple email addresses for each customer - to use depending on the type of form.

example:

[email protected] to send sales order confirmations

[email protected]  to send invoices

Is it possible to set the up so one or the other is selected depending on the type of form??  (or even better yet - code it to the form regardless of the type).

Thanks in advance!