Hello y_haas,
Sending all clean messages to a second destination would be to use your SMTP routes and set the default to a different destination.
So anything pending for your quarantines, should be routed to the appropriate quarantine - whereas anything else not quarantined/dropped, will fall into this default SMTP route to go to your second destination/mail server.
NOTE: This is not creating a copy, this is re-routing the original email to the second destination/host.
Your second destination must be ready to accept this traffic.
Regards,
Matthew