Yes, you can share books between both Kindles BUT if you're both reading the same book at the same time, you may want to turn off Whispersync, so that you don't lose your place in the book.
Whispersync allows the Kindles to sync with other Kindles on the same account as well as all the Kindle apps on the same account so that you can jump from one to the other without losing your place. You can turn it on an off via your account
here.