There are too many variables involved in thinking about students with multiple disabilities or with severe disabilities to allow for any generalized description of the educational impact upon students.
The type of disabilities will greatly impact the types of support services that the students will require. A student with a severe hearing disability will require different types of accomodations than a student with a significant loss of vision, but the procedures used to teach either student will need modification. A student who has a profound intellectual disability will need another set of modications.
Students with multiple disabilities will require adaptations to instructional methods to address the special needs of all their conditions. The combinations of disabilities that a child has will need to be assessed, prioritized if necessary, and accomodations provided to address the presence of multiple challenges.