you should be able to handle that via a custom target mapping.
Just copy the default one and change Email field to e.g. Email2.
Then when preparing a delivery, choose this target mapping:
Adobe Campaign Help | Target mappings in Campaign
Note: you won´t be able to mix that for different recipients.
e.g. for user 1 use email attribute and user 2 use cusEmail2 field, that would need different deliveries then