Slices export overlap

Hi everyone ! 🙂 

I have been searching for weeks about something and I'm questionning if it's technically possible or not.

I have an artwork composed of 2 images put side by side (front view and rear view of a mannequin).

I created one slice for each image (equals half the width of the artwork) ; so I can export the front, and the back as separate images.

But on top of that, I need a 3rd rendered image of the combination of these 2 (so the full artwork basically). But when I try to add a 3rd slice and place it over the extra 2, the other ones disappear.

So it looks like I can only render it full artwork size, or the 2 pictures individually ; but not the combination of the full artwork + the 2 pictures.

I have to use a image merge tool for now. 

See all 3 pictures below.

Any help is welcome ! 

Thanks