I've always been a little bit confused about some parts in The Dark Crystal, even after reading extra content about it that wasn't shown in the movie or the series, but I think I finally understand this scene now. If you don't like spoilers, then you probably shouldn't read this, even though it was pretty obvious, but The Dark Crystal is great representation of what it means to be dual. With the Skeksis being the dark counterpart to the Mystics and the Mystics being the light counterpart to the Skeksis, combined together, they form the urRu, ethereal beings yet shown to be a little more mundane with their vital mistake.
When it was (kind of vaguely) described how the urRu split into two, I always still wondered how. I needed a visual of what exactly happened. It took me awhile but I understand now that isn't supposed to be viewed in a physical manner. Their division was supposed to be viewed metaphorically. The reason why the urRu wanted to be split into two forms was because they were uncomfortable and disturbed by their shadow. They thought that if they could take out their dark side that they would be a pure being which is highly relatable but still a foolish way to view yourself as an individual, but they had to experience that in order to see that your "pure side" is just only that, and your "Dark side" is just only that as well, but combining those two together is what makes the urRu whole. They were never supposed to be one or the other and their shadow, no matter how dark it may be, it doesn't define them completely, they are still these majestic beings that are appreciated in Thra.
To add onto this, I've always found it interesting that they added astrological references in what's supposed to be a kid's movie. They mentioned that there was a "great conjunction" happening during that time period. I don't know how astrology works in that universe since all of this is fantasy but the great conjunction from what I understand is actually a real transit where Jupiter is "touching" Saturn which then creates this massive change ("some good, some bad" - Aughra). Of course a catastrophic change happened in the series but I just want to point out the details in other areas. I'd like to think that both the Mystics and the Skeksis are represented by the planet Jupiter. Jupiter rules over a lot of things and I feel that those two split beings represent the fine line between the good and the not so good when we think of what Jupiter means symbolically. The Mystics represent the spirituality and philosophy that Jupiter offers while the Skeksis represent all things that relate to indulgence, which is also ruled by Jupiter. When the two are met by Saturn, the planet of authority and discipline, everything that went wrong in that world was then corrected and restored back to its balance.
