... my opinion
... my opinion
I believe that if the 2 busses can ever be paralleled, even if there's a closed transition between the breakers, and the other one then drops out, the downstream system needs to be rated to withstand both sources.
Even though the duration is short, that's the time there would be a higher probability of a fault, during the transfer to another system.
The only way I would consider only one of the sources (that being the higher rated system) would be if there was a Kirk-key interlock that absolutely prevented them from ever being closed together.
This is a big issue with critical power delivery because there are always multiple sources to provide backup or bypass operation. And my experience is if the two can come together, the electrical system must be able to handle the combined fault curent.
Because, the consequences of a combined fault contribution onto an underrated bus can be catastrophic - even life-threatening.
Perhaps if a real-world scenario made it essential that two systems meet during some kind of emergencty procedure, and combine on an underrated bus, there could be a procedure that isolates human contact from the equipment during the act. That's the ony way I would consider it. And I would have that procedure laminated and posted to assure compliance - or at least to limit my liability.
JM