Once you aquire those AP Eggs (3x ability points earned) Mastering Dresses doesn't take that long. The effect doesn't stack, equipping two on the same character is no better then one but each girl can use one. It only takes a third of the time to master abilities. The only jobs it doesn't work for are Floral Fallal, Full Throttle, and Machina Maw. For the purpose of mastering jobs faster it makes sense to assign one job to each girl, but once mastered there's no reason to keep Yuna in a support role or Paine as a combat character.