You can simply add another server.
ssh-test(config)#tacacs-server host 1.1.1.1
ssh-test(config)#tacacs-server host 1.1.1.2
ssh-test(config)#do sh run | i tacacs
tacacs-server host 1.1.1.1
tacacs-server host 1.1.1.2
You can also configure the time out for TACACs.
ssh-test(config)#tacacs-server ?
timeout Time to wait for a TACACS server to reply
ssh-test(config)#tacacs-server timeout ?
<1-1000> Wait time (default 5 seconds)
Hope that helps.