If I understand the task well, always_bcc is not your friend.
I would try the transport daemon (transport table) instead.
Of course, we suppose we want to forward to the same user name.
For the updated question:
If you want to send copies, you can use the virtual alias transport.
and in regexpalias do something like this:
after the usual postmap regexpalias matching mails should go for both destinations. You must be cautious if you have other virtual alias sources defined at the server, because virtual aliasing will be done only once, and exits at the first match. For example if you have:
And in myaliases you forward firstname.lastname@example.org to email@example.com, then your regexp will be never reached and single mail will go to firstname.lastname@example.org.
Because you are forwarding the mails directly (transport), and those servers supposedly wont sent the mail back, no more virtual aliasing happens, so there should be no mail loops.