Yeah, of course shadows with < 100 opacity will always add to each other if they overlap. I can't think of an easy way to improve on the current simple demo/tutorial without creating other problems or failings (with my limited skills, of course). Is there an easy fix you can think of or that you are hinting at when you say "should fix that" 
mindfaQ - any suggestions would be appreciated?
Before writing the tutorial I had a good look at Pode's 2D Dynamic Lighting: but it doesn't produce shadows as far as I understand (and I'm happy to be corrected) and it requires the use of a 3rd party plugin (unsupported by Scirra in the event of an update causing a crash) and it requires the use of additional software that, as a hobbyist, I'm not familiar with nor inclined to learn how to use (I?m still learning how to use C2 when I?m not distracted by my day job).
Thinking as I type - I suppose if the ground had a uniform fill colour with no texture effect then each shadow could be made from a darker shade of the ground's colour. Then shadow overlaps would not affect each other. However, that solution would obliterate any ground texture and, therefore, would be a viable alternative but with a huge trade off. Also, in situations with diffuse lighting in the real world, shadows can overlap and 'add' together - they are rarely exactly the same shape as the casting object unless you are very low to the ground... The same applies to situations with multiple light sources.
When you mention that this only strictly applies to objects with a rotational symmetry about the z axis - I agree, the tutorial only demonstrates shadows cast by such objects. If you want to create complex shadow casting objects then I can?t think of an easy way to do it in C2, the only (and long) way I can think of would be to create a LOT of differently shaped shadow sprites and then select the appropriate one according to sun position and object angle. However, although I haven?t tried it, I imagine that you?d be on a hiding to nothing because it would be very hard to implement and still wouldn?t be as good as anything Unity could produce.
I agree with your last comment: if you want a realistic 3D shading solution for your game, don?t use C2! But we?ve got to struggle on with the tools available...