It is rare in real life that a drug only divides over one volume. A drug is divided throughout the body and the volume of distribution depends upon where the drug distributes and where the concentration of the drug is measured. The concentration is most often measured in plasma, as this is the most accessible body fluid.
As a mathematical exercise, put 12 units of drug in so that it distributes evenly into the plasma and interstitial layer of fluid. That means that the drug distributes over the plasma and interstitial layer with 3 and 9 units respectively. Now determine the volume
of distribution of the plasma fluid layer. This would be equal to 3 units/3 litres or 1 unit/litre. From this it follows that the Vd1 = 3 / 1 = 3 litres. If we determine the concentration in the interstitial layer then this is 9 units / 9 litres or 1 unit/litre. From this it follows that Vd2 = 9/1 = 9 litres.
See how drugs can distribute over different components and what this means in the human body by clicking here and selecting the body compartment model and the human body model (module 2 and 3).