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If it's what you're going for thematically, that their duality leads to mutual destruction, then by all means do it. As long as the story builds to that, there's no reason not to do it. If you're ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/38412 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If it's what you're going for thematically, that their duality leads to mutual destruction, then by all means do it. As long as the story builds to that, there's no reason not to do it. If you're just doing it as a twist out of nowhere (which according to your description, doesn't seem to be the case) then it will leave readers dissatisfied. But as-is, it's fine. Best of luck.