I have also had this issue on a single system with dual power supplies and and the only choice I could come up with was to use the power cable on one side only and only notify.
One system to one UPS, the other system to the other UPS. With dual power supplies and feeding different UPSs and each of those being on different circiuts, I would not worry about powering off the System i but would want to get notified as soon as possible so I could manually shut down the systems if needed but there is no other way to my knowledge to tell.
This at least would tell you for sure if you got notified by both systems at the same time that major power is out to the building completely.
You could possibly take it one step further and if notified check the other system and see if it was notified and if so issue the powerdown to both systems. But that is a lot of work. I hope you are up to the challenge.