Answer is quite simple in term of principles then I'll let you look at details to determine how this can be implemented:
- for services that rely on accounts, Zentyal is only able to use "local" repository (you can't have one single Zentyal managing accounts on which other Zentyal will connect, e.g. for authentication)
- you can synchronize multiple Zentyal servers, in term of account content, either using LDAP layer or Samba 4 layer but not both.
- this means that once you have deployed at least one single DC somewhere, if you need to deploy "file sharing" (AKA Samba) on all Zentyal servers that will need accounts.
Zentyal is initially designed as "all-in-one" box. This new (to come with 3.4) HA stuff seems to bring something slightly different but I'm totally lost with features is can really bring. Better wait for someone from Zentyal staff to provide you with more accurate answer regarding this