There is no soft edge mask in flash although as was said it can be faked.
I guess you could create a layer above the photo with a not too wide box that had soft edges and was the same color as the background. As it moves on you wipe off the first image below it and on hte other side you are wiping on the other photo. Havent tried it but it might appear to be a soft wipe.
You might also make the image wider than it needs to be and make the edges faded out in photoshop. I know flash supports PNG files which support multilevels of transparency. So I guess you could overlay the two photos on top of each other in flash, assuming the 24bit png files appear correctly.