After successful installation of Windows Server 2012 (R2) you might want to join it to the first server - usually already installed as a Primary Domain Controller - i.e. you might want to join it to the domain (for example as a secondary Domain Controller or for any other reason imaginable).
To accomplish this (with minimum effort) I'd advise doing the following procedure:
1. rename the second server before joining it to the domain (specially if you plan to use it as a secondary domain controller - if you rename a server that has been promoted to a domain controller, things are bound to go wrong with the Active Directory etc.). Restart the machine when requested.
2. set the network adapter and try pinging the first server just to make sure that the network is working (in order to eliminate it as a cause to possible problems which could occur later on).
3. for testing purposes (i.e. if you're sure what you are joining), it's advisable to disable the Windows Firewall (or other firewall already installed) for the network used because it can cause some issues (and you shouldn't need it anyway).
4. enable the Network Discovery feature. Namely, if you try to join to the domain without it enabled, you'll probably succeed in the second try (the first will probably end up showing up an error telling that the domain controller can't be contacted, but after that you can simply click "OK" and give it a second shot), but that could cause a lot of problems (with the primary domain controller discovery, while searching the allegedly joined domain etc.).