i wonder how you managed to get the texture (back package) into Live3d. I am maybe wrong, or missed something... but didnīt u setup/map the background illustration with the "stencil-channel-trick" (i am talking of the option click method to get that automatic little tile, without loading a perfect matching stencil map)... IMHO, this mapping style does not come into Live3D. How did u work around this issue? Did u convert the extruded shape, or UV-burn the texture? I had no success with such a "stencil-ed" texture.
You are correct. The back side of the packaging does not have a second map applied to it. You would need to make the front and back sides of the packaging separate objects as I'm pretty sure there can only be one map per object. (I'll double check on this).