Flatten to both FG and BG at the same time

Sometimes I like to use both the FG and BG layers on a map for textures, but I also want to flatten everything for shading. So far, the only to do this with stamps on the border between the FG and BG is to copy all of them, place them on top of each other on a pixel perfect basis, then flatten half on the BG, then half on the FG. Aligning everything on big maps, particularly when using light effects, can be very tedious and extremely laggy.

I think it would be useful to have a one-click function to tell the app to copy EVERY stamps on top of each others, and then flatten half of these on the BG, the other half on the FG, in order to easily work on flattened maps using both layers.