redundant servers

[RYAN vAN GINNEKEN]

Ok here is my scenario i have 3 registered domains and two Freebsd boxes 
running bind 9 postfix 2 and apache 2.  These servers are on separate 
networks server one is master for itself and slave for the remaining 2 
domains on the server 2.  Server 2 is slave for server 1 and master for 
a couple of its own domains yet only has one ip address.  This i seem to 
have figured out over the past couple of years what is confusing me 
right now is how do i get the servers to back each other up i mean  if 
one goes down  the other one takes over dns web page mail sever 
everything.  seems like a great concept but how do i achieve this i 
think i got the dns stuff  but what about the rest dns seems to be 
everything in these situations so  i hope this is not to far off topic