We often read in grammar books, that before measure words one should not use 二. Instead, we should say 两 Hence, 二本书 would be incorrect. One should say 两本书.
But I have come across two cases in genuine Chinese learning materials where 二 is used before the measure word.
- 二斤半苹果 :two and a half jins apples
- 你们二位: you two
Are these sentences grammatically incorrect?