Uwe,
The e-mail 2 field is just for storing and managing additional e-mail addresses. It does not have any special powers, although e-mails received from that address will be auto-resolved to the contact.
If you wanted to have something that would CC the e-mail address 2, that might be kind of tricky, as you don't have a lot of time between when the e-mail is created and sent in which a plug-in or workflow could act--basically once the e-mail is saved it is sent. You would need something that would run synchronous that would intercept it before it was sent.
One thing you could do is create a custom workflow or plugin that runs on create of an e-mail that sends a copy of the e-mail to the address in the e-mail 2 field.
You cannot do such a workflow with the workflow wizard as it does not allow you to specify which e-mail address from the contact the e-mail is sent to.
