Are there any known cases of an individual, who was both blind and deaf since infancy, that was able to function on a similar level to Helen Keller? I personally wasn't able to find anything and was wondering why. Have the techniques that Anne Sullivan (Helen Keller's teacher) developed been expanded on, or even applied at all, to people with similar disabilities?
I wrote this answer a couple of months ago about Julia Brace and Laura Bridgman, two DeafBlind women who attended school in the US prior to Helen Keller.
There have been many other DeafBlind people educated around the world, such as Victorine Morriseau (1789-1832), Alice Betteridge (1901-1966), Robert Smithdas (1925-2014)...there are many, many more. There are many DeafBlind people around the world today doing amazing work, like poet John Lee Clark, linguist Jelica Nuccio, artist Alan Constable, athlete Taranath Shenoy, and many more. Many have advanced degrees, such as lawyer/advocate Haben Girma.
I am not sure what you mean by the techniques Anne Sullivan developed - tactile fingerspelling existed before her work, in fact she learned it at the Perkins School where she and Laura Bridgman had both been students. Tactile lipreading, known as the "Tadoma method," also originated at Perkins. They have indeed been used by many, many other DeafBlind people, before and after Helen Keller.