Jim Williams from Linear Technology explains how to battle switching regulator residue in linear regulator outputs. If you want to clean up the switching regulator output by using a linear regulator, you’ll run into difficulties.
Linear regulators are not very good at suppressing high frequency noise, and switching regulators output loads of it. The “secret sauce” to suppress the high frequency noise is to add ferrite beads on both the input, and the output of your linear regulators. More about this is covered in the app note.