I like the concept of a creature flipping sides like that. Also, damage and healing; cool.
However, the mechanic doesn't work (as is): You turn Yin over to your opponent's Yang on your turn. Your turn ends. It doesn't do damage. Your opponent turns his Yang back over to your Yin on his turn. His turn ends. It doesn't heal you.
I guess what you could do, is make Yin's ability free, whereas Yang caries an (other) cost. That way it makes some more sense in terms of mechanics.
You turn your Yin into Yang. It doesn't to damage.
On your opponent's turn, it has summoning sickness. It heals you.
Next turn he switches into Yin. It doesn't do damage.
Your turn it does it damage, but has summoning sickness.
Because it's sort of in stasis, it balances out the effect, so it's nowhere near OP, but it still works.