Join them to make a shape with mirror symmetry.
This puzzle was inspired by a remark in a draft of a slide presentation by Donald Bell. It implied that two identical pieces cannot form a worthwhile symmetry puzzle!
Fraternal Twins was featured in the September 2020 issue of the Russian science magazine Квант.
Here is a set that I made from wooden cubes, glue, and acrylic paint:
Brian Menold of Wood Wonders Online offers an elegant hardwood version of Fraternal Twins for sale.