Cement board over existing drywall dilemma
Yes!  This is actually a very common install method.  I do drywall, plastic, then hardieboard on nearly half my tub shower surrounds.  It is easier to work with 1/4" HB and I get flatter walls.  You could make a point that this type of install is the most effective and long term in a lot of cases.  For a stand-up shower I would however put something like redgard over the backer.