does shag area rugs look good on berber carpet?
No not at all I would know I'm a carpet salesman. You would have to go with a low pile area rug but a shag would look ridiculous!
I'm trying do decide if you are serious or not... Absolutely not, this would be a horrible combination. If you want an area rug over a berber, go with a more typical low-cut persian rug or similar. It will add to the decor rather than subtract from it like a shag would do. It will also be easier to walk on if it is in a traffic area.
area rugs on top of carpet? this should be ok if area rug is contrasting in color. this is a way to add color, and to divide space in an open floorplan. one concern is that sometimes area rugs placed on carpet tend to bunch up, there is a product at carpet padding suppliers that is a sticker for area rugs on top of carpet.
You'll be hard-pressed to find anyone who lived through the 70s and has any fondness for shag. It's something best left in Brady Bunch reruns.