[Dovecot] two mailboxes - sieve?

[Egbert Jan van den Bussche]

Hi helpful list!

I have this user that has two different accounts om the mail server. One 
is a system account, the other is a virtual account (for what it is 
worth...). This user would like to have all his email to the virtual 
mail box (maildir format) automatically moved (or delivered) to the 
system mailbox (also maildir fornat). Both are on the same machine and 
use the same postfix/dovecot installation. Something tells me that I 
might need the sieve plugin for lda here since dovecot is the local 
delivery transport. I have no knowledge whatsoever about sieve or its 
scripting. What do I need to o apart from enabling the sieve plugin in 
dovecot.conf?
Or is there another simple solution for this problem directly in Postfix 
or with fetchmail? A simple working example would be wonderful!

TIA
Egbert Jan (NL)