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			That's easy when you use calibre's book editor. I don't know ebooks-convert. 
In the editor, you can split the ebook anywhere you like, using the preview screen. 
 
Normally, the editor will automatically detect where to split the files while importing a docx file, but it needs indicators (like a new chapter). But it's easy enough to do it manually, when the auto-detect is not to your liking.
		 
		
	
		
		
		
		
		
		
		
		
		
		
	
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