Continuing to experiment with A Young Man in a Hurry.epub.
Open the file you want to split and go to Code View and go to the location where you want the split.
Insert
Code:
<hr class="sigilChapterBreak" />
at the split point and press F6 [split].
The following appears in Book Browser:
Code:
A Young Man in a Hurry_0010.xhtml
A Young Man in a Hurry_0010_0002.xhtml
The naming of the file is just right and if you don't like the split point do a Merge With Previous and redo.
This is what should happen for any split.