Adding multiple TACACS+ servers for auth

Currently we have Fortimanager setup using 1 TACACS+ server and would like to update that since we have more then one server for redundancy. When I went to add see if I could add a secondary server under System settings > Admin > Administrator, I noticed there is only a drop down for one server which is pointing to our Remote Auth Server TACACS+ (which is pointing to one node). If I go to add a remote auth server for Radius I notice there's an option to add a secondary server. I'm wondering if the only way to have automated fail over to a backup remote AAA server is to use Radius? My only other thought is to add multiple administrators each with the same accounts pointing at different TACACS+ servers. Any help would be greatly appreciated.

Thanks!!