AES Message Authentication Vulnerability
If this is simply the AES permutation on a single block, it's hard to find such a pair.

If it's AES-ECB with multiple blocks, you can pick each block from either (x1,y1) or (x2,y2), producing a new message that contains parts from each of them.

With other modes it depends on that mode, but with many modes there will be a similar mixing attack as for ECB.