The only difference when the rug is going to be used over a carpet is to make sure that the color of the rug complements the color of the carpet.
How to lay an area rug over carpet.
When laying the rug over carpet however the concern is more about the rug wrinkling or rippling which presents a big tripping hazard wrinkling will occur because as one area of the rug is stepped on it will shift assuming it is not properly.
Likewise if you have a plush style of carpet installed choosing a low profile berber or woven rug is your best bet.
On a hard floor an untethered rug is more likely to slide or slip but when layering area rugs over carpet the rug is more likely to wrinkle or ripple.
Area rugs generally work best over short pile or looped carpet.
Rugs should always be anchored whether used on a hardwood floor or on top of carpet.