https://en.wikipedia.org/wiki/Tandem_Computers
"Tandem's NonStop systems use a number of independent identical processors and redundant storage devices and controllers to provide automatic high-speed "failover" in the case of a hardware or software failure. To contain the scope of failures and of corrupted data, these multi-computer systems have no shared central components, not even main memory. Conventional multi-computer systems all use shared memories and work directly on shared data objects. Instead, NonStop processors cooperate by exchanging messages across a reliable fabric, and software takes periodic snapshots for possible rollback of program memory state."
The general ideas of failover for high-availbility go back much further.
https://en.wikipedia.org/wiki/Failover
"The term "failover", although probably in use by engineers much earlier, can be found in a 1962 declassified NASA report. The term "switchover" can be found in the 1950s when describing '"Hot" and "Cold" Standby Systems', with the current meaning of immediate switchover to a running system (hot) and delayed switchover to a system that needs starting (cold). A conference proceedings from 1957 describes computer systems with both Emergency Switchover (i.e. failover) and Scheduled Failover (for maintenance)."
Also worth a mention that the well known Paxos distributed consensus algorithm was described in a 1989 paper by Leslie Lamport, and the almost identical (but less well known) predecessor Viewstamped Replication in a 1988 thesis by Brian Oki.