I'm not sure if there is a way to already do this...
I found myself editing the veil nebula and really wanted to be able to apply two masks to an image to isolate the nebula. A really simple way to do this would be to create a Lum mask and clip in HT. Apply to image, then create a star mask and apply that to the image to further mask off the areas you dont wan to touch.
I'm guessing that this could be achieved with pixel math, but that is beyond me at this time. It would be so simple (UI) to drag and drop the first mask, have that tab visible, then drag and drop the second mask. You could then independently remove and invert those masks depending on what you want to do for edits. I find myself wanting to do this more and more frequently. It sounds simple to do, but maybe it is not that simple when executing this in the program.
I'm not a programmer, and only an amateur user of PI so simple execution of a concept like this would really be helpful. Does this already exist? Kudos to the programmers on this program... it is just awesome! Thank you!