"Why" would you do this is a better question then" how effective" in my opinion. The purpose in a cover is to create more value than the competition on a similar product. Do you want the book to look better that the books around it? Two things sell very well. "NEW" and "Remarkable." If you think a double cover can look like something that's not being done in your market than do it because it's NEW, or if the book looks so good that everyone will want to pick it up and talk about it, then yes, do the double cover because people are making remarks. I would consider textured papers as well or matte paper with Aqueous spot colors to get a more dramatic effect, along with a tremendous cover design.
Best wishes.
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