There is no reason you should ever do this
Shapechange gives you the magical ability to change form as a free action every round for as long as it lasts. You can keep changing from one thing to the next on every round. Shapechange does everything polymorph does, and quite a lot more besides.
When you cast polymorph, you change once, overriding any other changes you have. This form would follow the rules of polymorph, e.g. no Supernatural abilities. If you have not yet used your shapechange-granted opportunity to change this round, you could immediately override the change from polymorph with another change from shapechange, as a free action. If you have already used your shapechange change this round, (which would have been overwritten by polymorph) you’d have to wait until next round. Either way, your shapechange change could even be used to change to the form you already have from polymorph, but now with the extra features that shapechange offers.
But since you could have just taken that form, with additional features, by using shapechange and therefore not wasting any actions or spell slots, there was no good reason to cast polymorph in the first place.