There aren't. Zell23 and Zell999 are the same person.
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has the author of FOBS and Shiratane Tower(?) as zell23, zell999 is just a different username/handle for the same person. In short, my understanding of this tale over the years has been - FOBS was a passion project for Zell, that eventually stopped being fun, so they ditched it around the time that Succubus Affection was in development. Zell partners with someone called Ansara(?) for SA and other games, and things are fine for a bit. SA releases, Zell and Ansara begin working on other stuff, but come to a head when it comes to how they want the games to be going forward. To my understanding, Zell wants more of a focus on femdom/monstrous characters, Ansara wants things more balanced and human. They still work together on projects like SA2, and Shiro no Yakata. However, Shiratane Tower (or Tower of Shiratane? Google translate fails me) is Zell's personal project it seems, and is much heavier on the monstrous, femdom, and horror aspect than Shiro no Yakata has turned out to be with Ansara's input, though I haven't been able to find any info on that project in a bit.
Most of this info was gathered from that zell999 blog, but some of it has come from the Succubus Diary ci-en page or this forum thread. My info is probably not up to date exactly, but I figured I'd give my understanding of this whole situation.