During this outage, did the controller show the 1st RADIUS server as out of service for those 70 minutes or was it continually trying to use it? You mention you have two servers pointing to the same AD source; in this case there is no reason to enable fail-through since they hold the same database; in fact in 6.4 (not sure about other versions), you cannot enable fail-through on server-groups for dot1x networks without termination (which you also say you don't have enabled).
Run the following command to determine if the controller realizes the 1st server is unreachable (the out-of-service column), or look at the server group in the UI.
show aaa server-group summary
If it is showing out-of-service it should go to the second server. If it is not doing so, please pass along the AOS version and the results from the following:
show aaa server-group <name-of-group>
show aaa timers (to check on how long it keeps an out-of-service server "dead")
show aaa authentication-server radius statistics