First off, this isn't any of the Witch Mountain movies I'm thinking of. I believe it was from the late 80s or early 90s. There's a brother and sister who might be mentally disabled, and I think they have enlarged heads, but when they get close to each other, especially if they touch their heads together, they suddenly get really smart and/or have psychic or other mental powers. They might also be aliens who only resemble humans. I think they're intentionally kept separated to prevent the manifestation of these abilities. It seems like they both live in a nursery together, in a house with regular parents, and are treated like children, but they're fully grown. I think the setting was suburban America. I can't remember the exact tone of the movie, but it may have tilted more towards humor and the absurd rather than serious.
It looks like Slapstick of Another Kind, which is based on the novel Slapstick by Kurt Vonnegut (1976). It was released in 1984, and it seems to satisfy your requirements.
Caleb Swain (Jerry Lewis) and his wife Letitia (Madeline Kahn) are called "the most beautiful of all the beautiful people" by the press. However, when Letitia gives birth to twins who are called "monsters", the family doctor, Dr. Frankenstein (John Abbott) informs the parents that the twins won't live more than a few months. The Swains decide to allow the twins to live their short life in a mansion staffed with servants, including Sylvester (Marty Feldman).
Fifteen years later, the twins (also played by Lewis and Kahn) are still alive. They have large heads and appear to be mentally retarded. Their parents, who have not seen them in all those years, receive a visit from the former Chinese ambassador who informs them that their children are geniuses who can solve the world's problems. [...] A series of tests reveal that there is a telepathic connection between the twins, and their intelligence is only functional when they are together. Furthermore, when their heads are touching they reach a level of intelligence that has never been surpassed.
Their parents, fearful that incest may be prevalent, separate the two. They become despondent without each other, and the Chinese ambassador appears again to tell them to seek each other out. Once united, a spaceship appears and reveals that they are really aliens who were sent to Earth to solve all of the planet's problems