This KALLAX Connector will allow you to join KALLAX units together in several orientations. I use them when I have two units stacked vertically to make a 4 slot high by 2 slot wide arrangement. These connectors are designed to produce a large locating force using the existing countersunk holes on the units. The plate is designed to be stiff enough that there is no play available between the units as any movement risks shock loads.

It is recommended that you print these at 80% or greater infill to maintain a high margin of safety. No one wants a shelf falling on them.