"Multiple disability" refers to a situation in which an individual is faced with more than one type of disability. A student who is hard of hearing and has attention deficit/hyperactivity disorder, for example, would be considered to have multiple disabilities. The Individual Education Plan for such a child would need to include plans to address both areas of disability.
"Severe disability", on the other hand, refers to the severity of the disability. Saying that an individual has a severe disability would indicate that the degree of limitation being experienced is quite significant. Persons with severe disabilities require extensive supportive services and accomodations to achieve as much independence and development as possible.