It's highly recommended that you don't examine anything across the gown.
Untie the upper gown knot from the back and do the CVA tenderness.
Important points as you do the CVA tenderness:
Explain to the SP that you are going to pound on his back to see if there is any tenderness around the kidney area.
Lean your head forward as you do it to make sure that you look at the patient's face while doing it. Therefore, it's better to do it with one fist hand and not with two hands as it's often taught in med schools.
If the case is both respiratory and abdomen it's better to do back (respiratory + CVA) then front (respiratory) then tie the gown up again then ask the SP to raise the gown up for abdominal examination.