“Using the same mask in both modules is a great solution” - well, there is no other solution when developing an area with multiple modules 
they MUST be the same
As regards the Mask Manager - I also used it on the same photo, but in other areas, where it helped to eliminate masks overlapping issue.
Thanks for telling how to blend two shapes within the same module - I did not know that - that could have shortcut the way to the solution.
With that being said, hope someone finds this thread useful. We have collectively evaluated Darktable scenario for applying quite popular fading effect.