These celebrity couples have experimented with swinging, open marriages, and living in separate homes. Will Smith and Jada Pinkett Smith; Dolly Parton and Carl DeanCredit: Matt Winkelmeyer/Getty; Courtesy of Dolly PartonThe secret to a long, happy, and healthy marriage? For some celebrities, it’s about embracing a more unconventional relationship.Boy Meets World actor William Daniels and St. Elsewhere actress Bonnie Bartlett, Hollywood's longest…