Does Unity Create Multiple Atlases if I Slice a Texture > 4096x4096?

Say, I have a 6000x2000 image. If I slice it to two 3000x2000 images inside Unity's Sprite Editor window, when I build the game, would Unity create two textures for this one image or scale down the 6000x2000 image and create just one texture?

Inside the Unity project hierarchy, it obviously remains as one over-sized texture with two sprite definitions. But is it the same after I build the project or does Unity automatically optimize the textures?