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I'd count it as two. #YourChallengYourRules

I would count them as they were originally published. If they were originally published separately then count 2. For example, with ebooks lots of series are subsequently published years later under a single ebook. I have a few going myself atm. If you have selected this combined edition as what you have started reading then as you mentioned Å·±¦ÓéÀÖ will count it as a single book.

But if the 2 books was only ever published under one joint cover then it should probably be counted as one. This last is not common I think.

I think that would be considered 2 books. At least I always list them that way in challenges.