Some people have said, no to carpet on carpet but others have said why not! I am not of those, why not kind of girls.
In our current place, we are renting a older 850 square foot apartment. The carpet itself it okay, the flooring underneath the carpet is uneven and hollow. In the place we lived before I had laminate hardwood flooring and found the perfect rug for only $125! It was a steal of a deal... a mis-communication between co-workers (they sold the last old version of this rug I had on hold) and in the end they gave me this 8 x 10 rug worth over $1000 for only $125 (Pottery Barn isn't as expensive as you think... sometimes).
Isn't it pretty?
Before I bought this carpet I had a green area rug from Ikea. But I went paint crazy and painted my living room green. It was pretty obvious I had too much green and that is when I bought the PB rug. When we moved, I knew the place had carpet and since we couldn't paint, the old apartment was going to be in need of some color. The white shag wasn't going to be the best idea for carpet on carpet due to the fact the rug is very thick and the carpet was already a light color (also I knew my baby would be starting solids soon and spit-ups wouldn't be just white anymore). I still loved the green rug and got it out of my parents attic.
I wanted to touch on another point when it comes to area rugs. Every designed has a different option but I am going to quote Sarah Richardson on this one. On her first episode of Sarah 101 she said, "at least the front two legs of each piece of furniture should land on the rug. You don't want your area rug to seem like an island floating in the middle".