TLDR: The purpose of the secondary DASH address is to provide an extra way to recover access to your funds, in case you lose access to your private key to your primary DASH address. The secondary DASH address must reside in a different wallet than your primary DASH address.

The secondary DASH address is optional. It is a "backup" way to authenticate yourself in case you lose access to the private key to your primary CrowdNode DASH address. It is therefore essential, that your primary DASH address and secondary DASH address are not part of the same wallet as this would defeat the purpose of the secondary DASH address.
For example, if your primary DASH address is in your "wallet.dat" file of your dashcore installation and you forget the password to unlock this wallet or if the file is corrupted, your secondary DASH address won't help you if it's in the samme wallet file. For this reason, your secondary DASH address must be in another wallet than your primary DASH address. For instance, one could be in a hardware wallet and the other in a dashcore wallet.