The shutdown can fail to initiate when the outlet group is powered off on one UPS and a critical event occurs on the second UPS.
PowerChute Network Shutdown v3.1
Redundant UPS Configuration on any operating systems supported by PowerChute Network Shutdown v3.1.
This is a software issue. It only occurs for an outlet group power-off event.
If the UPS (as distinct from the outlet group) is powered off when the critical event occurs, PowerChute detects this and starts an OS shutdown after 10 seconds if another critical event occurs.
Update to PowerChute Network Shutdown version 4.x