Hey @Sebastian-Gonzalez ,
I think the terminology Radish Logic is using might be the cause for confusion. In Windows NT, there were primary domain controllers (PDCs) and backup domain controllers (BDCs). But since Windows Server 2000. Active Directory Domain Services (ADDS) has been multi-master, meaning all domain controllers are writable (other than RODCs, which is a different conversation). When you add a second domain controller (DC), it should not be referred to as a secondary DC. It is simply another DC, equal to the first. Either DC can authenticate, apply group policy, etc.
As for DNS, this depends on the type of zone you create. When you install DNS on a DC, you are just creating a new DNS server. The fact that it is on a DC just gives us more options when it comes to zone type and zone replication and zone security. If you choose Active Directory integrated for your zone type, then you can choose to automatically replicate this zone to other DCs. This option can take advantage of AD being multi-master, and have multiple writable copies of a zone. If you choose standard primary for your zone type, then you will only have one writable copy, and you will need to set up secondary zone(s) for fault tolerance.
You need to provide fault tolerance for both DNS and ADDS. But remember, they are separate services, and while they can work together to provide a solution, both need to be planned and configured.
Hope this clears things up a bit. Let me know if you have more questions.