It's hard to imagine what a duet between Olivia Newton-John and Gladys Knight would sound like, but such a thing is possible this weekend, when both stars will be guests of honor at the 500 Festival Parade. Newton-John is the Grand Marshal; Knight is a “legendary honoree”; both will wave from well-appointed rides and attend all the major galas (Snakepit Ball, 500 Soiree) and the race. 80-some floats , marching bands, giant inflatable balloons (Miss Piggy, Papa Smurf, a golden dragon, Cat in the Hat) and bagpipe choirs will march down the two-mile route, with an expected 300,000 spectators looking on, along with TV cameras for national (NBC Sports Network) and local (WISH) broadcast.