They both reached people with disabilities. But, Thomas helped those with hearing impairments. While Dorothea helped prisoners suffering from mental illness.
longer answer:
They were both part of a movement developed in the early 19th century when many institutions were created aimed for people with disabilities. Gallaudet found the first institute for the deaf and brought what we today know as sign language to the US. He was inspired by a religious revival that set him on a course of service and against oppression. On the other hand, Dorothea Dix began a campaign to help people with mental illnesses after seeing the awful way they were treated at jails. Her pleas pushed the government of Massachusetts to create separate facilities to people with mental illnesses.
