You can edit the users .qmail file by hand, and from there set it to forward to several addresses, unfortunately in its current form will be overwritten if you latter modify the account via the web manager.
It all depends how often you want/need to do this.
Via the web Id be looking at the aliases addon as a possible way I guess. Just be carefull not to create a loop.
I tried then with 2 addresses in the create an account and it seemed to send to a bad address, I guess ill hear from the postmaster soon about that address:)
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Damien