You have to kill the enemies when the raid event happens, if you dont kill the enemies; it continues until thy is done.
We have never had so far multiple raids simultaneously. Not sure if that's a thing, I hope not. You can have raid after raid in short time but at the same time don't believe so.

I also believe the raid has some sort of "maximum" time limit where it force shuts down if the raid is not completed or the enemies are not able to destroy anything anymore.