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?