Both phones can have iMessage, all you need to do is on one of the phones, go into settings, iMessage, then click on "receive at" then add another email address that isn't one associated with your apple ID, once the email is added and verified via an email that apple sends you, you can click on the original email address (your apple ID email) and remove it. Then you can both enjoy iMessage, and not have to read each other's messages all of the time.