You are correct that contradictory statements are not allowed on the Official GMAT questions.

One thing that you can always count on is that the two statements must be able to be true at the same time.

So, for specific value questions - such as "x = ?" The same answer must be possible for each. It is not allowed for one statement to give you "x =4" and the other to say "x = 5" There must be room for the two statements to be true at the same time.

For yes/no questions like this one, if one of them answers only "yes" then the other must at least allow for a "yes" as well. So statement one could have only "yes" while two allows for "yes" and "no." What is not acceptable is what we have here with a "yes" from one statement and a "no" from the other.

Nice job spotting this!