I feel like the conventional wisdom is that the two letters of recommendation should represent both humanities and STEM. Is this a universal best practice?
Does it matter what kind(s) of colleges a student is applying to?
Are there situations where a student would be best served by having letters from the two instructors who know them best, even if those fields are History and English, OR Biology and Math?
The college will not ask you to read between the lines. Unless they say specifically that it needs to be one humanities and one STEM, you are free to choose. I would still opt for different academic areas though - math and bio is fine, 2 different math - not so much.
Read the admissions web sites carefully for the colleges to which you are applying. MIT wants one from math or science and one from humanities, but Notre Dame only wants one LOR and it must be from among your junior or senior year teachers.
MIT wanted one of each. My older son obeyed, and then used those two for all his schools. His Latin teacher was his humanities recommender and his physics teacher the other. He was applying as a CS major where they asked. I’m sure she wrote him a fine one, but I wondered later if he’d been better off with a science and math for some of the other schools he applied to as Latin is not a class where you write papers. Since his history and English teachers liked him, but often objected to his minimalist approach to papers he was reluctant to ask them. It’s all water under the bridge now. He got into some great schools and got rejected from some great schools.
Anyway, read the rules and really think about who really knows you, likes you and gets you. My younger kid got B+'s in math, but still got a fabulous recommendation from his pre-calc teacher.