Sorry, I'm not sure what you mean. Of course you can have multiples in the same city, but I don't understand your comment in regards to powering facilities.
In case I'm not being clear:
Take a city with 5 factories, requiring 40 power total. If you have 2 power generators generating 60 power, you could assign 2 to one PG and 3 to the other. If you built 4 power generators instead, could you assign 2 factories to both PGs 1 and 2, and 3 factories to both PGs 3 and 4?
You can, when a power generator has its full 30 power available, link it to another power generator for a combined 60 power output which can be distributed as you like.
You can not power a facility halfway with one generator.
I'm not sure about your example, though. With a single factory requiring 8 power none of the conditions you list require more than the 30 power per generator. :/
Yes you can power facils from more than 1 source.Most of the time this happens inadvertently with our beloved new power assign system but it can be done manually also
Anyone that has gone to a city to try a fix a mysterious power shortage has probably experienced this the first go
If you are asking if it is possible to power facilities redundantly, I believe the answer is yes. But as Tey said, you can't use two generators with insufficient power left individually to combine to power one facility.
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