I found this searching online
We have just purchased two Kindles for our high school library as a pilot, and I found out a couple of things. First, you can have up to 6 Kindles tied to one Amazon account, and, if you buy a single title, you are allowed to put it on all 6 of them.
....so you and your wife would have two kindles, but one amazon account. The same book could be on both Kindles.
Anyone else know more? I did a search on the Kindle support pages, but didn't find anything one this.