This page provides some details on the settings available for the Translucent Two-Sided Material in V-Ray.
Front material – The material which is going to be used for front-side faces as defined by the object normals.
Back material – The material V-Ray will use for back side faces as defined by their normals.
Translucency tex – Determines which side (front or back) relative to the camera is more visible in the rendering process. By default this value is 0.5, which means that both the side facing the camera, and the one facing away from it, will be visible to the same degree. When this parameter is closer to 0.0 the more of the material facing the camera is going to be seen. When it is closer to 1.0, the more of the back material is seen.
Multiply by front diffuse – When enabled, the translucency is multiplied by the diffuse of the front material. This option is not available when the renderer is set to CUDA.
Force single-sided sub-materials – When enabled (the default), the sub-materials will render as one-sided materials. Turning this option off is not recommended.