I've done that in the past to several movies and it causes significant sync issues starting at the append point. It's been well over a year since I did it, Does now work?
I've done it with the Hobbit series and never had an issue. Rip's audio and video have to be identical, and the chapters of the second have to continue were the first one left off. Say first one has 23 chapters, second one has to start at chapter 24, 25, 26, etc... I renamed them with MakeMKV prior to the conversion. I've read it can be done with mkvMerge, just never done it myself so won't be able to tell you how.
HTC M9