I just looked over the discussion of interleaving in chapter 3 of Make It Stick: The Science of Successful Learning (really excellent book by the way - HT to zedzed for the recommendation). The authors describe interleaved practice as switching to a different topic before each practice is complete (pg. 65). That doesn't mean switching every practice question, as you say, but rather practicing one skill just a few times, then switching to a different skill (pg. 50).
Also, you can achieve flow (from my understanding) while still switching between subjects / skills / techniques within an overall subject. Let's say you're practicing baseball. You might practice batting, then catching, then throwing. I don't think you lose flow when switching from one to the other, as long as you keep practicing baseball.
My comment earlier assumed that interleaving also requires switching overall subjects every once in a while. But now that I look over the chapter, I don't see that mentioned at all. So maybe you don't even need that.
It's possible, perhaps just not for me or the person who wrote the article.
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