The way I have always run OS X server is to partition the boot drive. Then I use Carbon Copy Cloner or diskutil to keep an up to date clone of the server on the other partion. All shares are on a separate hard drive. Should one server 'side' fail, I can boot into the other and carry on. This method obviously does not protect against catastrophic hardware failure, i.e. the whole machine dies. I have off-site backups for this. However, in all the time I have run OS X server, I have very rarely had a complete hardware failure. I have however, had to go into the site and boot into the other volume quite a few times. The method you are suggesting would be problematic for DNS and I can't see a workaround for that.