What you want to maximise is overlapping surface area and glue.
How you get there will depend on your CAD skills.
One option is to print a 3rd sleeve that you push both parts into. Another option is to cut a 1.5mm deep slice out of the top face of the left part, and a 1.5mm slice out of the bottom of the right part so they slot together with as much surface area overlapping as possible.
What you want to maximise is overlapping surface area and glue.
How you get there will depend on your CAD skills.
One option is to print a 3rd sleeve that you push both parts into. Another option is to cut a 1.5mm deep slice out of the top face of the left part, and a 1.5mm slice out of the bottom of the right part so they slot together with as much surface area overlapping as possible.
https://youtu.be/FzCm1iTf8PA might be helpful, but all the examples are about quite chunky parts.