From what I have heard, you have to recite a minimum of three ayah's from the Qur'aan in one rakaah. I'm sorry I haven't got a fatwa.
E.g. If you read Surah Yaseen 1 ruku in the first Raka, then go on to read SUrah Mulk 1 ruku in the 2nd Raka, I don't think there is a problem with that.
Wa Allaahu Aalim.
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