How to play double-stops?

As far as I know, a double-stop means playing two notes simultaneously instead of one. Again, according to my limited knowledge, the higher of the two notes is the one written in the melody and the lower one is the added spice. Please, correct me if I'm wrong.
My question is: How do I know which note to play as the added lower note?
My question is: How do I know which note to play as the added lower note?