SSS is not evaluated in indirect rays traced by the SSS. Indirect rays fired by the SSS engine from one object that ends up hitting another object will return black. This is in part legacy inherited from the point-cloud based SSS engine, and in part a fundamental limitation of diffusion based BSSRDFs (which their inventor, Henrik wann Jensen, always warned were only exact for infinite, semi-infinite media and not for curved/thin surfaces etc). We have investigated how to fix this limitation of BSSRDFs but so far our efforts have not been successful. A workaround is to connect a Standard material to the diffuse attribute of a Rayswitch shader.
In the example below, you can see that green light is bounced from the cube on the left onto the sphere. However, no light is bounced from the cube when a Skin shader is assigned to it.