C# how do I combine images (textures)?

I have a character that has eight materials on it (I’m allowing the textures/colors to be swapped), but the eye/mouth textures (and a few others) could be layered upon each other without the pixels overlapping. Is there an easy a way to create a new image, in game, by layering the two images on top of each other; or would I have to go pixel-by-pixel?