I was looking for a safe and effective way to join two Kallax shelves for game storage. By using the existing, robust (and long) screws IKEA uses to combine the major, exterior shelves, you can safely combine different Kallax units together. 

This design has a small ridge to help align and keep the pieces from shifting/sheering once screwed down. 

Print in PETG to provide elastic movement rather than PLA's sudden breakpoint for safest option. Use 100% infill and rotate infill 45* each layer to get layers in all directions. The connector isn't thick, so this is fairly quick to print: 5meters, 13g filament. I printed in PETG and could not break or split it even with pliers, so I believe it's at least as strong as cheap metal. 

Included in this are covers for the screws (they look more finished covered by the same filament that the connectors are made of). I also included a simple spacer: as you can see from the pictures, the center of the Kallax shelves don't line up the way the ends do, so I put a simple spacer between the units in the middle to help distribute the load and prevent potential sagging by the shelves. 

PLEASE be sure to anchor your (now stacked) shelves to the wall so it doesn't tip over.  The included anchor hardware is appropriate and sufficient.