Splitting the image matrix into two matrix based on a condition

I have two parallel lines from edge detection. Now I want to measure the distance between each upper pixel of the above line and the lower pixel of the below line. So is there anyway I can do this. My plan was to create a new matrix for the above line and the below line and then find the difference between these two lines. It would be really helpful if someone can help me with the code. As I'm new to MATLAB.