Who is The Twins from The Man Who Mistook His Wife for a Hat and Other Clinical Tales and what is their importance?

The twins, John and Michael, have been in and out of institutions all their lives. They have also been set up as a type of sideshow on radio and television because of their extraordinary ability to tell the precise day of the week a specific date will fall on—even if that date is 80,000 years in the future. Sadly, John and Michael are separated by well meaning doctors. Although the twins are able to hold jobs and live apart from one another, they loose their unique numerical ability.