One controller is a single point of failure. A second Synology can share the domain so a dead NAS no longer means a dead office - here is when that earns its cost.
Synology Directory Server supports adding another Synology NAS as an additional domain controller for your domain - including, in current versions, a read-only option for branch-style use. The accounts replicate between the controllers, so either box can authenticate users. This page covers what that buys you, the one rule that limits it, and the moment an office should spend the money.
Survival of the login service. If NAS01 fails, NAS02 still authenticates users, serves DNS for the domain, and applies existing policy - the office keeps working while you repair or restore the first box. It does not automatically save the file shares: if the dead NAS also held the only copy of the files, the documents are still down even though logins live. Pair the second DC with replicated or backed-up file storage, or accept that it protects identity, not data.
Additional controllers only work with domains created by Synology Directory Server. A Synology cannot become a controller inside a Windows Server domain, and a Windows Server cannot become a controller inside the Synology's domain. Your redundancy lives entirely inside the Synology family. For offices committed to the Synology route that is fine; for offices planning a Windows migration soon, it is a reason to skip this investment and read the migration page instead.
The second NAS must be a supported model with DSM current, a static IP, and its clock and DNS in order. In Directory Server on the new NAS you choose to join an existing domain as an additional domain controller (current versions also offer a read-only domain controller mode), give it the domain name and domain administrator credentials, and let replication pull the directory across. Afterwards, hand out both controllers' IPs as DNS to clients - a PC whose only DNS was the dead controller cannot find the surviving one, which quietly defeats the whole exercise.
Synology also offers SHA (Synology High Availability): two identical NAS units acting as one, with automatic failover of services. SHA protects everything on the box, files included, but demands matched hardware and stores one logical copy. The two-DC route protects identity across two independent boxes that can even sit in different rooms. Offices usually pick: SHA for one critical all-in-one box, second DC when identity must survive and files already have their own protection.
Roughly: when a morning without logins costs more than a second NAS. Signs you are there - 15+ staff who cannot work offline, a single NAS already carrying files and the domain, no IT person on site, or a compliance promise about availability. Below that, a tested Hyper Backup restore (previous page) is the honest proportionate answer, and the money is better spent on the backup target.
The video builds the first controller - watch it, then use this page to decide honestly whether your office needs the second one yet.
Watch Full VideoNo. Additional controllers for a Synology-created domain must be Synology NAS units running Directory Server. Windows Server DCs and Synology DCs cannot be controllers of the same domain in either direction.
If clients were given both controllers as DNS servers, logins continue against the second controller and most users notice nothing. File shares that lived only on the dead NAS are still unavailable - the second DC protects identity, not the files themselves.
A second DC is an independent controller replicating the directory - it protects logins. SHA pairs two identical NAS units as one logical device with failover for all services on that device. They solve overlapping but different problems, at different costs.