texture behind another texture google vr skybox

Hi, I'm new to google vr cardboard. I'd like to put a texture on the skybox of my google vr application (a landscape), and behind it another texture (an image made by me, with spots of different colors). When the user see a point on the landscape, and on the image behind, at the same coordinates, there's a particular color (for example red), something will happen. I don't know how to put the second texture behind the skybox texture, and how to get the coordinates of the point of view of the user.

Thanks