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A "sequel" that takes place during the events of its predecessor is called a midquel (or more precisely, an "intraquel"). In your case, only the first 25% of your second book is intraquel, but the ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/46007 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
A "sequel" that takes place during the events of its predecessor is called a midquel (or more precisely, an "intraquel"). In your case, only the first 25% of your second book is intraquel, but the point is: not only is this allowed, but it's so common there's even a term for it. To give you another example, that's probably more relevant to your own story: _Shrek Forever After_ begins during the events of _Shrek 1_, with Fiona's parents being informed that she's been rescued from the tower just before they were due to sign a deal with Rumpelstiltskin. This is what sparks Rumpelstiltskin's vendetta with Shrek, and sets up the main storyline. So if your second book needs to be part-intraquel, go ahead and make it part-intraquel.