Even though it's ultimately all about balance, you are right in them "canceling" each other. Some focus on increasing the energy in the weak ones, and some are more about reducing overactive ones. Your body will end up saying, "what are you trying to do to me!?!?"
As I understand, you can use them on different days, or even different sessions in the same day. Just as long as your body is able to handle the work.
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