Calibre requires you to select all files to be merged AND specify the target (defaults to the first).
This is different from the way Sigil does it 1) A single file selected Assumes Merge into previous (in list) when you ctrl-M. 20 If multiple selected, then it want a confirm (still first)
viva the difference. I use both, because of DIFFERENT ways of doing things (and I don't want either to conform to the other)

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