?! or !? which one do you put at end of sentence?

I assume you mean that the person asking the question was very upset or angry, and you want to add the exclamation point to show it. It may not be in the grammar books, but it is done sometimes. The question mark seems more integral to the sentence and is usually placed first, I think.

"What did you do?!" he screamed.

(I'm trying to agree with you, but personally I would forget the exclamation point. It doesn't seem necessary.)

He may well have exclaimed; but his statement was still a question. You are only allowed one form of punctuation in a correct sentence, the doubles are seen when texting or informally communicating with friends, that's when you see all the faces and stuff ^.^, don't you know!

So, it correct as, "What did you do?".

Neither. It depends on the question; use one or the other.