My wife and I each have an iPad 2. When we want to read the same book we both have to pay for it trade iPads. Is there a way to download a single purchase to both iPads or at least to "loan" the book to the other one?
Based on what you have described, it appears you each have your own AppleID. Adding each other's Apple ID to the device would permit this. There is really no legal and fictional way to "lend" books otherwise ( at least not iBooks purchases).