MQTT (was WiFi) interference when using two nodeMCU devices
My bad!

Although my symptoms were consistent with WiFi interference, the interference was actually at the MQTT level.  

The root cause was that I hadn't realised that each MQTT client must have a unique client ID. When the 2nd nodeMCU connected, it disconnected the first nodeMCU from MQTT, which then went into a reboot sequence to recover, which then returned the favour to the other nodeMCU.