I dont think you can do that. The secondary radius server in the list will only be used if the primary goes down. If i were you i would pick a site or talk to your Cisco rep to get a vMX trial license that you can use to test the new radius server. Simply create a new network, add the client vpn config and create a new anyconnect profile you can install on test clients. When you are happy with the result you simply change the radius server on your production network.
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