Ignore Scott's advice. In general, organising photos isn't one of his strong points.
So, forget the idea about putting your images in one folder. It's unnecessary. And yes, you can have two main folders, with subfolders beneath them.
However, the advice isn't completely off-target and its real point is to avoid overcomplicating your folder system. Don't have some pictures on this drive, others on that drive, others....oh, and the 2009 vacation are still on the old laptop. Don't get anywhere near that kind of patchwork organisation. Instead a simple system of two main top level folders, and then a third next year when you need another disc, means that at any moment you can say all my pictures are under control, they're all recorded in Lightroom, I know they're all backed up, and I waste no time finding ones I need because they're all in Lightroom. So if Scott's "one folder" advice makes any sense, it's about keeping your photos in as few places as makes sense.
John