Animation with foreground and background image

Hi
I have an image with 2 people in the foreground and the background is a busy street. I would like to have a transition effect where firstly, the 2 people in the pic slowly appear and when that part of the image is fully visible, I would then like the background to slowly appear. I have tried cutting the image in two (foreground (with people) and background (the busy street) in photoshop, however when the transition is completed there is a white line around the people (where I separated the image). Does anyone have any ideas on how I can get this done with the final image looking as it did originally?

Thanks