Take up two equipment slots?

Trying to figure out a way to make a custom backpack take up two item slots. I want it to take up the bag slot and the vest slot at the same time. I've tried multiple things but am unable to do it. I can make it so it only goes in the vest/bag slot or so it goes in both but not at the same time. Is it even possible? Anyone have some tips for me?
 
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