I found this by accident.
I was streaming and couldn't figure out why my duplicants wouldn't use the sink properly.
When finally I hovered over this sink, while having it's info panel open, I realized that the two directions presented were different as you can see in the picture below.
This works on the SINK and the WASH BASIN:
Left (Selected) = Both (Dupe Use)
Right(Selected) = Left (Dupe Use)
Both(Selected) = Right (Dupe Use)
I went through both sink and was basin, and every time i changed them, this worked.