how to automate public key authentication when server dual-boots two linuxes

I have a hopefully simple question on setup for sshd.

I recently switched my server from rshd to sshd and it works fine.  I am 
using public key authentication.

However the wrinkle is that this server has two different linux partitions 
which I need to alternate between (strictly only one at a time, not a 
virtual machine setup).     With rshd they both appeared to be identical as 
far as rsh authentication.   With sshd, when I booted the second one, and 
then tried an ssh to it from another machine, of course I got

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!
 ... rsa fingerprint ...   etc etc.

I realize that this is exactly what sshd is designed to do, to detact 
impersonation of the server, but unfortunately that's exactly what I need to 
do (but of course don't want any other impersonator to be recognized).

I could try just overwriting the second server partition's
/usr/local/etc/ssh_host_rsa_key.pub
with the first one - but is that the right way?   Would that cause other 
problems?      Hoping someone else has come across this and there is some 
recognized solution to this.

John