Then take an iron and set it to a medium heat.
How to get rugs to lay flat on carpet.
Double sided tape could be used in place of the carpet tape.
Many times this will allow the rug to lay smooth for a while.
This should flatten the areas out.
So the bottom line is that yes you can put an area rug over wall to wall carpet.
However for rugs that are nowhere near furniture in the center of a hallway or foyer for example the carpet to carpet non slip pad is your best option for preventing ripples in your area rug.
Cut tape to fit each edge of the rug.
Apply carpet tape around the edges of the rug.
With the towel still covering the carpeted area hold down the iron and press.
Turn over the rug and lay it on the carpet.
Turn over the rug with the right side down.
Lay a damp towel over the edges and iron them flat.