Each witch would watch watches until their watch stopped working. Then they would watch their watch until they can't watch because they cannot remember which watch is which. Then they set off nukes that blow up the entire universe in one giant whichwatch of watching witches watching which watches they choose.
Which watch each witch would watch depends on which watch they want to watch. Is there a difference between the two watches? Ask the witches. They seem to love watching watches. Or, maybe both of the witches like watching all watches (If there is more than two that is )
The standard deviation of witches watching which watch, is around 4.3 and we must calculate the mean assuming that the watch that the witch has is around 3 years old. This yields an answer of 7. Magic has worked, I am now quite proud of myself. All hail the Sith!
Which witch watches which watch depends on the perspective. For example, if witch 1 watched watch, how could we know if the other witch was also watching the watch? We could not ask her, because witches notoriously lie. Therefore if it's unclear if both witches are watching the watch, it's irrelevant to ask which witch was watching which watch.