Normally, in Blender, ‘material’ refers to the base color you apply to the faces of an object, and although you can apply many materials to an object, you can only apply one material to a face. ‘Textures’ can be added on top of these base materials, and in this case you can have as many textures as you want, one on top of the next, like you’re wanting.
You’re new so I assume you’re probably still just trying out the Blender Render engine and not Cycles. In that case, you need to assign the second texture to the same material that you did with the first texture. Do this in the texture panel of the properties editor, and then go into the UV editor and load that texture into the same UV map that you created for the first texture. That should be all that’s needed.