Once you develop film in E-6 for slides, there is no silver left in the film to do any more developing. All the light-sensitive material is bleached and fixed out leaving only a dye image.
Same goes for C-41. Once you develop film in C-41 you are left with only a dye image, no silver left for further manipulations.
Therefore, a film is "either" developed in E-6 or C-41. Yielding a slide or a negative respectively. If you develop C-41 negative film in E-6 slide chemistry you get a positive. This is called cross-processing.
If you develop E-6 slide film in C-41 color negative chemistry, you get a color negative. This is also called cross-processing. People do this for a unique "look".