Thanks. Yellowarden and Crimsonic started as most of my figures do; a fun concept, sketched and refined. Their transformation were the result of many other transforming figures I've designed, of knowing what works and what doesn't with Shapeways' materials. Their combination, however, was the result of many iterations. A basic idea of how the figures would interact, tested and tweaked; often one figure would need to provide what the other couldn't and vice versa. Remnants of this can still be seen in the combined mode, the rounded feet appear to be Crimsonic's, but are actually part of Yellowarden. Ultimately, a pair of combining figures needs to be a pair, designing both figures simultaneously allowed eaiser combination and a sturdier combined form.
Perhaps the most intriguing aspect of combination isn't just making it possible, but making it
feel possible. Transforming figures can be rather intuitive on their own, but for two different figures, their combined form isn't as obvious. In that sense, the biggest challenge was making the transformation simple and easy to understand; to combine, Yellowarden is partially dissassembled, before each section is transformed. Even though combination does require 39 steps [ as shown here:
https://www.tfw2005.com/boards/threads/yellowarden-crimsonic-combined-form-shapeways.1094072/ ], manipulating smaller sections is simpler, both to do and to communicate through instructions, resulting in a more accessible design.