The ACS doesn't work like an ASA where you can have one active and the other standby. What you can do is to have one primary and another as the secondary, where depending on your TACACS+/Radius commands the authentication server role will change, for example:
tacacs-server host 10.0.0.1 key xxx
tacacs-server host 220.127.116.11 key xxx
This configuration means that the router will try to contact 10.0.0.1 always, but if this server goes down, the router will try to contact 18.104.22.168 until the 10.0.0.1 is up again.
So as you can see the ACS servers doesn't have control about which will be the active server, only which is the primary and which is the secondary (primary and secondary is important because you configure the settings in the primary and this one replicates those changes to the secondary).
Mauricio is right. To expand his answer, if you have several ACS appliances only one of them is primary and all the other ones are secondary.
"Primary" and "secondary" concepts are different from "active" and "standby" concepts. All ACS are "active".
The switch configuration tells the switch which ACS to talk to. It can be one, two, three, any number of ACS. Also if there are more than one ACS, the switch configuration gives the preference to the first ACS declared in the configuration. Only if the first ACS doesn't respond at all then the switch will try to talk to the second ACS declared. Only if the second ACS doesn't respond at all then the switch will try to talk to the third ACS and so on.
here's an example of switch configuration with three ACS