Hi bro, the separate system is like a quarantine tank, a fully functioning system. It is also like a backup system in case your main system fails.
For your main system, you dose to keep water parameters correct? However, when you do water change, don't throw away the water. This water is mature and have almost the same water parameters, so use it to change water for the back up system. This way, you can maintain 2 systems and save on new saltwater.
Everytime you water change it's like version 1.1, you keep the version 1.0, it's like saving files on computer.
Also, because the back up system has very similar water parameters, you can easily move livestock over without any acclimation, especially during emergency. Very easy to maintain because you are basically saving the water during water change. I use this for quarantine purposes.
Hope it helps.