[Exim] 2 MTA's on the same host

Can anyone tell me if it is possible to make Exim (listening on port 25)
to route *all* mail to another MTA running on the same server but different
port (eg 2525)? No mail will be local to the server on which the MTA's are
running. I need to 'buffer' mail received from external hosts on the server
and then feed it to the second MTA. This MTA can, occasionally, take some
time to do it's required processing so I'd like to release the originating
MTA as soon as possible, hence buffering it with Exim.

Can anyone give me any pointers? I'm using Exim v3.33. Despite everything
that I try all I get is 'remote host address is the local host' and a point
blank refusal to do anything more with it. I haven't been able to find
anything in the FAQ/list archive....

Thanks,
Ian