When instructors are share teaching and they have a difference of opinion on how to handle a test question.
Present to class as united. Take a class break,State to students the instructors will have made a decison by the end of next breatk and have instructors return to prep room to discussion options.
I think the best way to handle these discrepancies, is to prevent them. Even if you are co-teaching, the materials and tests should be reviewed by both instructors prior to sharing them with the students. If either instructor does not agree with some content or test material, they can come to some conclusion at that time. Once students are involved, it will always appear as if the instructors are divided if they need to confer regarding testable material. That being said, no environment is perfect. And if such a discrepancy arose, I agree the best way to handle it would be to discuss privately and make a unified front.