As they grow apart, they can't take turns playing TV's cutest kid- so one stays and the other must go

Twins Mary Kate and Ashley Olsen, who star in Full House, are being split up. After they turn 6 this year, only one will be needed to play bubbly Michelle Tanner. Because of stringent labor laws when the show first aired in 1987, producers were forced to hire twins to play the one role. That's not the case after the children have their sixth birthday in June. There is also concern that, since the girls are fraternal -not identical-twins, they soon might start to look noticeably different. The news has upset the cast of the hit ABC sitcom -especially John Stamos, who is the closest to the youngsters. "He really loves them," says one insider. "Everyone who knows John knows that he's always taking the twins to his house or to the zoo or Disneyland. John talks to the producers and writers every day to figure out ways to make sure that both Ashley and Mary Kate will be on the show come September." A source told STAR: "When Mary Kate and Ashley were younger, often they would do different scenes based on their moods. If Ashley was grumpy, then Mary Kate would do a scene that required a 'happy' Michelle. "The producers also want them to start memorizing their lines. The girl who is better at learning her lines will help to determine which one will continue as Michelle." Another source adds: "Jodie Sweetin (who plays Michelle's sister Stephanie) was 5 when she memorized her lines, and all of the other kids who are 5 learn their lines. Instead, Ashley and Mary Kate have their lines fed to them by a trainer." Adria Later, the twins' coach and on-set teacher, says: "Mary Kate stays a little more focused and will happily repeat her lines until they're perfect. Ashley, on the other hand doesn't understand why we need to continue once she's done her scene. After two or three takes, she'll announce, 'moving on!' " Whatever the decision, it could have a lasting effect on the girls. Dr. Carol Lieberman, a family psychiatrist and media script consultant in Beverly Hills, says: "Twins are normally competitive with each other, which is why the sister who may be dropped from the show will feel devastated, less intelligent and less attractive. The star of the show could feel that she acted in a devious, unfair way and had a role in hurting her sister. "Since Mary Kate and Ashley Olsen are fraternal twins, not genetic clones as are identical twins, the situation may be a little easier on them because their appearance will change as they mature and each will develop a unique personality. "That's why it's so important for the twins' parents to show both girls a lot of attention and make it a point not to favor one child over the other. The parents should seek out another activity or talent in which the rejected twin can feel special and shine. The twins need an environment tah will build their self-confidence." A source also adds that since the producers fear a minor "backlash" from the show's devoted fans if one of the girls is let go, they've talked about creating a new role. "Last season, there was an episode which had Michelle's cousin visiting from Greece," adds another insider. "Their cousin was actually one of the twins wearing a dark wig. "The producers and writers are talking about having this family move in next door to the Tanners. By doing this, they'll keep everyone happy. including their fans who have come to love the twins as if they were their own."