Direction of mass transfer in two film theory
Just keep in mind: the driving force of mass transfer is a concentration gradient. Nature tries to 'equilibriate' a difference in concentration between two phases. 

As such, the mass transfer in that sense will be from the phase of high concentration (S2) to the phase of lower concentration (S1). 

Two film theory is only a simplification of the mass transport directly at the interface, in which we assume there are two boundary layers on each side of the interface in which the concentration gradient is constant, giving us linear concentration profiles, and the concentration directly at the interface is determined by Henrys Law. This makes dealing with such problems much easier. 

Mass and Heat Transfer are often taught in one course as they have many similiarities, but Heat Transfer offers a more intuitive introduction to the principles of transport phenomena in my view:

In heat transfer the driving force is the temperature gradient. We intuitively know that heat transfer will be from the hotter system to the colder system, and formally know that from the 2nd law of thermodynamics. Can you see the similarity to mass transport?