I dunno if this is the best way, but it will work:
put the two dress levels on seperate layers, and put a new layer in the middle. Put them all in a folder or bone layer, and set its masking to 'hide all'
Select the top layer, and set it to 'do not mask'
Select the bottom layer, and set it to 'add to mask'
set the middle layer to 'mask this layer'
Now create a shape similar to the top dress layer, fill it with black, and choose 'soft edge' as its effect... then fiddle with the softness... you might also want to change its opacity, as well as set it to 'multiply'.
Is this drawn in AS? nice work with the textures... can we see a bigger screenshot??
