[exim] [Secondary MX] Total Newbie question on setting up

Hi...

I'm using since a certain time exim 4.x (we are on 4.63 and will be
upgrading soon), with no difficulty really, setting up RBL and other rules
for our personnal usage..
Yet i now wish to setup a secondary mx on a second server we have... (We
have had on primary server a 4 hours downtime, and saw that we lost certain
messages)

My problem is, i have found nowhere a tutorial on how to set this up...
I am therefore asking community for a clean "what to do" in order to setup a
proper secondary MX on the second server i have...

Must i install a second exim on this server ??? Must i change configuration
on both servers (in order for primary to receive messages) ???
What are the security advices (i have concerns on open relay servers, and
rbl non listing we have managed to cope up to know) ???

In short, your advices are pretty welcome, on how to set this up...

Yours

Tdldp