Use the Same Document in Two Locations?

Note that in my back matter for eBook and Print book, I have the same documents (e.g. Acknowledgments).

Thus, if I change the Acknowledgments, I must do it in two places. Is there any way to have a single file that appears in both locations?

Yes, indeed. Check out §10.1.5, Including Text From Other Documents, pg. 214 in the user manual for the documentation. You can link the content in from other items in the binder, or to .txt & .rtf files on the disk, synchronising content between different projects, which may be of use for that “About the Author” and “Also by…” sections.