Using the same mask in both modules is a great solution Alex. I read this thread and thought “it’s about time I learned to use the Mask Manager”. There you can create groups and do the blending you are looking for. Here is the page in the manual.
http://www.darktable.org/usermanual/ch03s03s05.html.php
A couple of things to note. You have to create a group in order to use the blending operators. You don’t have that feature if they’re just 2 shapes being applied by a module. Another thing is if you share shapes… when you make a change to it in another module, it’ll change the behavior in all modules that use that same shape. Sometimes you’re better off creating a duplicate in the mask manager and allowing for separate modules to have their own “version”. Depends on the situation. And 1 last point is that you can’t change the name of masks or groups in an individual module, but you can in the Mask Manager.
Looks like the Mask Manager will be this week’s topic on my podcast 