Splitting Epubs?

I know I did this once before. I just don't know how, I think it involved another app, but I can't find the right app.

I have a very long Epub file, too long for me ebook reader to handle. I don't know if it's a size issue or length issue. It has chapter titles included, but doesn't have any chapter breaks, so I can't use EpubSplit.

I need to be able to split it into multiple files. My current idea is to (a) duplicate the file and (b) remove everything extra from each section, so the first copy retains chapters 1-50, the second retains chapters 51-100, etc.

I can't figure out how to duplicate the file in Calibre. I *can* do this fairly easily by exporting from Calibre and duplicating in the OS X finder.

I can't figure out how to delete the redundant sections at all. I know I did this before, but I don't know how. Some Appls such as the Calibre Editor and Textwrangler show line numbers; I would need to remove lines 14275 through 53483 from the first part, etc. I don't know how to select and delete long sections of text. I can't scroll down, there's too much.

Any idea how to go about doing that?