Maybe have a look at the linux howtos like:
The domain name provider that you register a domain with usually operates two (or more) dns servers to (redundantly) answer queries. Most people do not host their own DNS servers, but simply configure or have their domains configured at their registrar.
If you’re really up to setting a freedombox up as a public DNS server then you would need to delegate the domain to the freedombox (a DNS entry at the registrar). For a second DNS server you would then of course need a second freedombox with a different IP. (Besides figuring out to properly configure “Bind” on the freedombox.)