It is unlikely that someone would have both diseases, but two things can happen.
1. That someone is thought to have one illness (like CIDP), but really it is an early presentation of MS. I have read of this happening several times and most of my family says this is the case for my cousin. Sometimes you do not know something is MS untill a second episode of strange problems or until there are clearly central nervous system changes.
2. there is a lot of overlap in these diseases–since both are demylinating, diffuse, fluctuating, mystifying….. As lots of people on this website state, often people with GBS/CIDP have things that classically are thought of as CNS (central nervous system) not peripheral nervous system. For example, the fussy brain, vision changes, fatigue, trouble with internal organ function, etc. My neurologist keeps thinking I might have MS or a spinal cord problem because he is not convinced that the atypical features that I have (especially the fatigue and vision changes) are from GBS/CIDP. Fortunately, lots of people on this site write about these as part of GBS/CIDP so it is clear that they can occur with these diseases and do not have to be from MS or a central demyelinating process.
WithHope for cure of these diseases