How do I show all UV textures at once? I am UV mapping my model and I don't think there are any problems so far. 

However, only when each object is selected it shows the texture.

I want to see textures on all objects, like hair, skin and clothing. But I can only see one at a time when I select it.

I'm using blender v2.79
You have to turn on Texture Atlas addon in Addons tab. 
It's a workaround, but there's noting better at the moment. Waiting for 2.8!
https://www.youtube.com/watch?v=cIok7SCQ3SA