Update 21 APR 2019
There is a newer easier-to-print version of this puzzle at https://www.thingiverse.com/thing:3578594.
This puzzle was my entry for the 2011 IPP Puzzle Design competition in Berlin where it was a Jury 1st Prize winner.
"Two open-ended Superstrings with the correct spin can be merged into a super-symmetrical cube."
The idea for this puzzle emerged when I was designing shapes based on two interlocked tetrahedra. Inspired by ideas from 'string theory' and 'super-symmetry', I began to search for shapes that were both symmetrical and as similar as possible.
The two pieces may look similar but they are very slightly different.
Taking the two pieces apart is not too difficult; reassembling them back into a cube can be more challenging. Hold one of the pieces still; move the other piece through the correct sequence of lateral and rotational moves.
The solution document can be found in ThingFiles.