Fill area between two overlapping shapes

Question

I am trying to convert a binary mask consisting of two lines to a mask that consists of the area between the two lines. However I am stuck and don't have an idea on how to move forward.

Here is an example of the mask I have: Mask

The goal is to make it look like this : Goal

I am using python and hope that there exists a library that manages this task. I tried plt.fill_between however I had no success with this. The masks consists of an numpy ndarray(208,208) The matrix looks like : Matrix

Thank you very much for your help