Multifake, in regards to the two examples you provided, they differ substantially on a technical level. The majority of Desiree's work is a mixture of getting the right image in-camera and a lot of work done in Photoshop. You're looking at hours of work in some cases per image depending on the original.
Erwin Olaf's seperation is a tricky one as it's more realistic and could be done in-camera as well. Photoshop can be used to achieve elements in the image although sometimes that may not be a practical choice.
Photoshop is a powerful tool that I'd suggest you start learning if you like the look of these images.