My Boomerang aircraft were designed by Bill Whitney and have FAR 23 type certificate as well as an Australian type certificate. This about the asymmetric flap requirement is one of the things told to me by Bill Whitney himself when we were discussing the 10 year cable replacement. The aircraft has to be able to be controllable in all three axes following a total failure of one of the cables, and to be able to maintain control with one flap up and the other fully down.
Point is that many of the "thou shalt not"s taught (eg I was told NEVER to touch the mixture control below 5000 feet?!) are not backed up as limitations in the POH, some may be applied by lazy instructors as a "one size fits all" eg sideslipping with flaps down, use of carb heat in the approach, (Cessna says to put it cold AFTER landing) CSU power setting tables, use of electric fuel pumps in Cherokees, the list goes on. Question everything.