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?
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.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."