3. Apply Multiply
Select all the feather paths, go to Transparency pallate, select Multiply.

Final
Now you can duplicate the feather and lower the Opacity (Transparency).

About the author:
Nick started http://www.ndesign-studio.com in 2005 where he showcases his work, now N.Design Studio is also a Illustrator community where you can find Illustrator tutorials and free design resources.