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Sigil also has a manual split file function where you put the cursor where you want the split and press Ctl-Enter. It duplicates the head and end stuff and makes the new files correct.

You can also go through the files and add a split marker with Ctl-Shift-Enter and they'll all be split when you later press F6. The split marker is merely an hr tag with a class that sigil looks for so once you add one you can copy it and use a regular expression to automatically add them. I use that to redo books from Project Gutenberg so that the files start on a chapter (h2), first merging them together with Ctl-M then splitting them in the correct place, just before the h2 tag.