You can install the DNS and DHCP services without configuring them and they will just sit there idle for the most part.
When the DNS service is installed on a domain controller it will automatically receive all the DNS domains that are stored within Active Directory. Your internal domain is probably setup in this way.
For the DHCP service it won’t do anything until you configure a scope for it to issue IPs. You have to be careful when doing this with two DHCP servers on the same network because if they are giving out the same IP addresses you’ll get IP conflicts. What you do for DHCP is setup both servers to serve out different ranges from the same subnet.
For example, if your network uses 192.168.0.0/24, and your DHCP scope is 192.168.0.100-192.168.0.200 you setup the existing DHCP server to issue 192.168.0.100-192.168.0.150 and you setup the second DHCP server to issue 192.168.0.151-192.168.0.200. Any machines which are already using IPs in the 192.168.0.151-192.168.0.200 range will need to be rebooted (or have an IPCONFIG /RENEW done on them) so that they request a new IP from the servers.
In the event that one of the servers goes offline then you’ll still have a server that can issue IPs until it runs out. Generally you’ll want to increase the network size when you do this so that each server has enough IPs to issue if the other server goes offline for a long period of time.