Cleveland arts groups have been ratcheting up efforts to improve the city’s cultural scene, but many have discovered that joining forces can be the most effective way to compete in a world with seemingly endless entertainment options. “So why aren’t the creators of two significant new Cleveland arts festivals working together?” The Cleveland Play House is planning a theater and arts festival for May 2006, but the creators of a Labor Day arts-and-technology festival will beat them to the punch by nine months. So why not let two become one? Well, for one thing, the Play House’s artistic director thinks that Labor Day weekend is “box office poison.”