9 times out of 10, it is not a good idea to put an area rug on top of carpet. It makes a room seem smaller and cluttered. Big no-no in my book.
Area rugs on top of carpet = no
Area rugs on top of hardwood = yes, especially if you are using the rug to define a seating area
Can I also just mention now how much I dislike carpet? I feel like I’ll vacuum it, then 2 days later it needs to be vacuumed again… it always seems so dirty…ick.