It could just be a way of keeping their careers seperate and not having to be compared (ie. western singers Kylie and Dannii Minogue for example).
Like I said, it could be (this would be for Kumi's case) Avex, or she herself, wanted a stage name of sorts. By using a different reading for 'koda', she gets to keep an essence of her real name (koda being one way to pronounce/read the kanji) but using different kanji in the written form.
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