Have the two MC on top of each other, and then apply tweens so that each one fades in or out at the same time as the others. You may need to have two lists built for the images, basically so one is the queue list and the other is the visible photo, then they switch.
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jeremy - gnomeontherun Educated questions often get educated answers, and simple questions often get simple answers.
There are different ways to do what you want, but as Jeremy said, the easiest way would be using tweens, which you could trigger either from the timeline or from AS.