It Takes Two

It Takes Two is a 1995 American film starring Kirstie Alley, Steve Guttenberg, Jeremy London and Mary-Kate and Ashley Olsen. Its title is taken from the song of the same name, by Marvin Gaye and Kim Weston, which is played in the closing credits.
Plot.
Two unrelated young girls who happen to look identical meet by chance. Amanda Lemmon is an orphan, and she is about to be adopted by the Butkises, a family known to "collect" kids. She actually wants her child-loving social worker, Diane Barrows, to adopt her instead. Diane would like to do so, but authorities will not let her because of her low salary and unmarried status. Alyssa Callaway is coming home from her school piano recital competition, only to find that her wealthy father, Roger, is about to marry Clarice Kensington, an overbearing self-centered gold-digger socialite who plans to send both Alyssa and Thomas off to boarding school in Tibet after marrying Roger.
The identical strangers long for the other's life and decide to switch places, Alyssa and Amanda tell Thomas not to tell anyone about the two switching places and he agrees. While Amanda enjoys Alyssa's wealthy lifestyle and Alyssa gets to experience being a kid at summer camp, the two get to know the other's parental figure and discover that Roger and Diane would be perfect for each other. Desperate to set them up, the girls arrange many "chance" meetings between Diane and Roger, hoping that they fall in love.
Upon having spied Roger and Diane laughing and swimming together in a lake one afternoon, Clarice manipulates Roger into moving the wedding up from the next month to the next day. Soon after, Alyssa, while posing as Amanda, ends up being adopted by the Butkises without Diane's knowledge, and is taken away by child services. Alyssa discovers the only reason the Butkises had adopted so many kids was to put them to work in their salvage yard.
Roughly two hours before the wedding, Amanda, who poses as Alyssa, proves to the family butler, Vincenzo, that she is not Alyssa. He contacts Diane and summons to have the real Alyssa picked up from the Butkises' salvage yard. In the meantime, Vincenzo, Thomas and Amanda work to stall the wedding.
Just as Roger is about to say "I do," Diane bursts into the church with the real Alyssa behind her, and Roger falters at the sight of Diane, recalling all the good times they had together. In that moment, he realises he has fallen in love with Diane and confesses this to Clarice. Furious, she slaps him and tries to do the same to "Alyssa," blaming her for sabotaging their relationship, but is stopped by Vincenzo and Thomas. As Clarice storms down the aisle, the real Alyssa steps out from behind Diane, and Clarice claims it's a "conspiracy" that there's "two of them." She tries to take this new opportunity to slap Alyssa, but Diane steps forward in time, barking at her to "Back off, Barbie," and calmly informs her that she has something in her teeth. Humiliated, Clarice moves to storm out of the church again, but Alyssa embarrasses her even further by stepping on her wedding gown, causing the skirt to rip off.
An incredulous Roger learns that Alyssa has been with Diane all this time and it becomes apparent to them that Thomas, Amanda and Alyssa had orchestrated their meet-ups all along, about which the girls are extremely smug. After some encouragement from the girls and Thomas, Roger and Diane kiss, and the five of them board a horse-drawn carriage, driven by Vincenzo, to take a ride through Central Park.