There are other topics about this, with proposed mock ups of two drop downs, but in practice there has not been much demand for this.
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if you have so many categories that selecting one is super complicated enough to warrant two drop downs, you probably have too many categories for users to understand anyway
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if you really need 100 categories, typing is still the fastest way to select them, a drop down becomes not so useful
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in places where people “need” a thousand categories, they usually mean tags, and we support tags, which are much better for lightweight ad hoc groupings of topics
Categories are like walls, build 4 and you have a nice house.. build 40 and you have a maze.