What is this challenge? Is it already made or are you trying to make a new ruleset?
Its more of a quirky gimmick actually.
As I wrote sometime ago on Reddit:
I think it would be interesting to make premade sims fusions and defusions, for example:
Francisca Vanderburg defuses into Francine Vanderquist and LaTrisca Olderburg.
Honey Darnell defuses into Holly Dartz and Caverney Channell.
BUT Francisca Vanderburg and Honey Darnell also fuse into Franney Vandenell.
But then again you could later on split Franney Vandernell into Fratis Vandark and Linney Pimpernell.
Or you could fuse her further, like with Anaton Willmoto, fusion of Anais Willheim and Leighton Sekemoto. Which then makes Franton Vandemoto.
Or other combinations, like a three way combinations between Star Shue, Gloria Goode, and Maggie Strong is Storia Goodrong.