Man I have a hard enough time with one mailbox, I dont think I could cope with two! :-)
Unity can support multiple domains but what you are talking about with two different email addresses will take two mailbox. Thus 2 licenses per user which might get expensive.
If you were OK with both messages going to the same mailbox you could have both extensions assigned to the same mailbox (alternate extensions) but the end user wouldn't know which number (6020 or 4020)the original caller dial.
Other than that all will work fine. And you would only need a single pilot and then you can use a key sequence to get to the other mailbox. (Messages button, *, mailbox number, password.)
Thanks,
Keith