[Exim] two exim on one machine - two queues?

hi there, I have exim on port 25 to receive smtp stuff,
I want another exim on 26 to stick all mail it receives into a file...
(a firewall will redirect certain hosts to that port)
the exim-26 has only one delivery which is appendfile and a router which
always selects it.
however.... both exims use the same queue, so if a message cannot be
delivered straightaway, because the appendfile
is locked it goes in the queue...
so how can I avoid exim-25 picking up a message from exim-26 in the
queue and deliver it to the outside?

thanks
conrad