Arnold Schwarzenegger has always wanted to do a sequel, called Triplets with Danny DeVito and Eddie Murphy as a third long lost brother. DeVito pitched script ideas back in 2003 but Schwarzenegger was too busy running for governor. DeVito agreed with his decision but doubted he would become governor.
The money was not available to pay to the two main stars and the director their normal fees. They decided to split a percentage of the gross profits between the three of them and ended up making more than they would have made taking their fees upfront because the film was such a hit.
Universal needed to know if Arnold Schwarzenegger, Danny DeVito and Ivan Reitman were committed very early on because they were under deadline. They made the deal on a napkin at lunch with agents; they signed it and DeVito then had it framed.
Throughout the movie, Julius tells everyone he meets that he is from an island (shown at the beginning of the movie) in the South Pacific 300 nautical miles (350 miles) SSW of the Fiji Islands. Although the island depicted in the movie is fictional, there is indeed a VERY small island 350 miles in the SW/SSW direction of Fiji known locally as Ceva-I-Ra. Ceva-I-Ra (also known as Conway Reef) is an atoll-type coral reef stretching just under 2 miles long with a tiny sand cay (with some vegetation) in the center approx. 2 hectares in area.
When Julius goes to his mother's old house and meets up with Granger (one of his and Vincent's six fathers), Granger's grandchildren are played by Jason Reitman and Catherine Reitman -- the real-life children of director Ivan Reitman.
Arnold Schwarzenegger, Danny DeVito and Ivan Reitman were all realistic and levelheaded about business and thought Twins (1988) might be a difficult sell. But when watching the dailies they knew the film was a hit after people laughed at six takes of a scene.
When Julius and Vincent passes each other outside the cinema. The marquee outside the Grauman's Chinese Theater lists the film Willow (1988). "Willow" was released in the United States on 20th May 1988. "Twins" was released in the United States on 9th December 1988.