No. Unless you do something dumb like "don't test that the failover system is fully functional before taking the primary offline;" this applies to any maintenance, which also includes upgrades. Usual HA tactics apply - don't rely on one power source, one network link, or just about any SPOF. (Which means that requirements start breeding like rabbits, which gets expensive rather quickly - whereas Github is free-as-in-beer, which is good enough for most people)
Perhaps the only case breaking both at once would be deliberate action (DDoS etc.).
Perhaps the only case breaking both at once would be deliberate action (DDoS etc.).