Combining Distance Maps
An obvious choice would be $\frac{d_1}{d_1+d_2+\epsilon }$, where $\epsilon$ is a small value to prevent division by zero.

Using your images, I get:

top = ColorNegate[Binarize[ImageApply[StandardDeviation, 
           Import["http://i.stack.imgur.com/aFNfB.jpg"]]]];
bottom = ColorNegate[Binarize[ImageApply[StandardDeviation, 
           Import["http://i.stack.imgur.com/Kipvo.jpg"]]]];
tDist = ImageData[DistanceTransform[top]];
bDist = ImageData[DistanceTransform[bottom]];

\[Epsilon] = 10^-10;
Image[tDist/(tDist + bDist + \[Epsilon])] // ImageAdjust