Bob and Walt are two conjoined-at the hip brothers whith total opposites. Thanks to their teamwork, being conjoined twins is not a handicap to them. But, when Walt's dreams of stardom of acting on the screen and Bob's shyness clashes, they both begin to fall out. This doesn't help when Bob's Internet girl arrives in town, unaware of their handicap, and when Walt gains his own TV show with Cher.
They are played by Matt Damon and Greg Kinnear respectively.
Conjoined twins Bob and Walt Tenor try to live as normally as possible. Outgoing and sociable Walt aspires to be a Hollywood actor, however, whereas shy, introverted Bob prefers the quiet life. They run Quikee Burger, a diner in Oak Bluffs on Martha's Vineyard, that guarantees free meals to customers whose orders are not completed in three minutes, a testament to how skilled and in sync Bob and Walt are with each other. Though Walt is comfortable socializing with women, Bob is the shyer of the two, and carries on a long-distance relationship with a pen pal named May Fong whom he has never met in person, and who is unaware that they are conjoined twins.
Walt gets a role in a local play. Bob stays as much as possible in the background, as he has a tendency to get stage fright. Following the play's success, Walt decides to follow his dream to Hollywood and persuades his hesitant brother to go along for the ride. They rent an apartment in California and become friends with fellow aspiring actress April Mercedes. When she expresses curiosity about their conjoinment, Walt explains that they share a liver that is mostly Bob's, and that because surgical separation entails risk to Walt, Bob would not consent to the surgery, even though Walt favored it. It is also the reason why Walt appears somewhat older than Bob. (Greg Kinnear is seven years older than Matt Damon in real life.)
Walt's efforts to find acting work in Hollywood are fraught with difficulty, and his agent, Morty O'Reilly, is little help, offering at one point to get him a job in a pornographic film. Cher is upset that she has ended up starring in a prime-time TV show called Honey and the Beaze. She wants out of the deal, so she decides to hire Walt as her co-star (since her contract states she can choose anyone she wants), certain the show will get cancelled. The producers, realizing Cher's scheme, foil it by going forward with the production, compensating for Bob's presence by keeping him out of the camera frame and employing bluescreen effects. The show is a surprise hit and Walt becomes famous.
Walt arranges for May Fong to come to California. Although he did this without Bob's consent, Bob and May Fong develop a romantic relationship, though the twins' attempt to keep their conjoined nature a secret proves challenging, especially since Walt must accompany the new couple everywhere, sometimes using creative solutions like disguising himself as a giant teddy bear. Eventually however, when May discovers the twins in bed, she concludes that they are a homosexual couple rather than brothers. Although Bob shows May that they are indeed conjoined twins, May is nonetheless in even greater shock at the deception, and flees.
Morty informs the twins that word has leaked about Walt and Bob being conjoined. Rather than hide this, the twins decide to embrace it, and they both become huge celebrities, making commercials and appearing on The Tonight Show with Jay Leno. While Walt enjoys this success, he knows that Bob is unhappy because he misses May. Resolving that Bob needs to be independent from him in order to be happy, Walt demands they be surgically separated. When Bob refuses, Walt, determined to convince him, begins acting wild and crazy until Bob can no longer take it. Walt starts to make a fool of himself by getting drunk and accidentally snatching a woman's purse. They eventually end up spending the night in jail for drunk driving; even though Walt was drinking, Bob is the one who suffers the hangover since their shared liver is mostly his. When they are released the following morning, they get into a fight and Bob decides to go with the operation.
On the night before the surgery, May shows up and apologizes to Bob for running out the way she did. Bob informs her that they're getting separated; although she does not want them to, he knows that it's best for them. At the hospital, May and April keep a vigil until learning from Ben Carson, a real-life neurosurgeon playing a cameo role, that the surgery was successful. Bob and May, both being small-town people, decide to move back to Oak Bluffs, but Bob finds the separation from Walt difficult, both practically and emotionally, and is unable to do the things by himself that the twins used to do together, such as maintain Quikee Burger's three-minute challenge or play hockey. Walt, for his part, loses his job when Honey and the Beaze is canceled due to low ratings, and finds it difficult to find subsequent work. He is also emotionally devastated by Bob's absence. After having a brief talk with Cher about what's best for him, he decides to move back to Oak Bluffs.
One year later, Walt and Bob are back in Oak Bluffs running the restaurant together, Bob and May have married and May is pregnant. The twins simulate their former conjoinment with Velcro clothing that attaches them to one another. Walt finds creative fulfillment continuing in local plays, including a musical in which he and Meryl Streep play Bonnie and Clyde.