Testing new server

I have an old machine I'm in the process of retiring, and want to test its
replacement. To do so, I'd like to send a copy of all locally-delivered
mail from the old machine to the new one to have it processed there. I've
set "default_transport = discard:Outgoing email disabled for testing" on
the new server, and tested single messages here and there, but am having
trouble figuring out a way to tell the old machine that its local_transport
should be both the normal local delivery and in addition to send a copy to
another server.

I did find always_bcc and the bcc maps, but I'm not sure if that's the
right answer. A transport map seems like the right answer, but that
appears to only have a single target.