A "mirror mask" that changes simoultaniously with any mask, to keep its opposite on a different layer or group

Many times a mask is used to separate a subject from a background. 
This feature will enable one mask for the subject to be connected to a “mirror mask” that will be updated inversely according to modifications to the original.
For example, if you reduce the contour of the subject mask, the “mirror mask” of the background is enhanced to the same pixels that were reduced from the subject mask. instead of updating 2 masks continuously,  a solution that never achieves the aimed precise separation.