Nested Compression Springs
A nested compression spring is a compression spring that fits inside of another compression spring to achieve more force. Many automotive valve springs are nested as well as suspension compression springs on railroad cars.
Obviously, one compression spring needs to be smaller than the other compression spring, but the other thing to remember is that they have to be wound in opposite directions. For example, if the outside spring is left-hand wound, the inside spring needs to be right-hand wound.
Another thing to keep in mind is clearance. Remember when a compression spring deflects, its outside diameter expands. Room needs to be allowed for the inside compression spring to expand.