When it's separately it's arousing because each of them knows the other is being spanked and as the spanker you know the same thing.
Each time the discipline is administered each has their own style of whimpering but they both, because of their relationship, have many of the same mannerisms and inflections in their voice. Not to mention their bottoms look the same, except one is older and one is younger.
It also always seems to drive the point home better when both are spanked together. Each of them knows right from the start that there will be no tolerating a defiant attitude from either of them.
The mother finds out that she has no special exemption from getting her bottom spanked and the daughter finds out the same.
It's a good thing and something that should happen more often than it does. This is a practice I fully support provided the daughter is over 18 years old.