I don't know how it wight work in the software you mention but maybe this will help and or give you some direction.

In Photoshop this is how I do it.
Bring both layers in, any format but they must be the same size/resolution to overlap properly.
As I write this I wonder, Maybe pngs won't work? Keep that in mind.

With the Shadow layer on top. It should be mostly white with some grays and darker areas, these are the actual shadows.
Now select that layer and change it to "Mulitiply".

In Photoshop, if you want a darker shadow you can duplicate that layer.
Or if you want a lighter shadow you can either reduce the opacity or use the eraser at say 23% to erase some areas.

Want to make your own shadows?
Make a new layer (on Top), also set at multiply and paint some grays.

Hope this helps.

Scott