[Samba] add a second file server for HA

[steve]

4.1.6 dual AD DC domain with a single 4.1.6 file server (fs)
Hi
We'd like to add another fail-over fs to our domain. Due to all the
conflicting (old?) documentation out there we would like to hear from
any user here who has implemented this _recently_. By implemented we
mean actually done it and are using it themselves. So,
1. is it doable?
2. could anyone list the acronyms we must use?
3. has anyone who has it working written documentation for others?

Any help most gratefully received and we promise that any information
leading to a successful implementation shall be documented in plain
English the moment testing has finished.
TIA,
Steve