This is also a problem for my wife and I. Wife has 2x iPhone (one personal and one her employers) with 2x numbers, 2x Apple IDs. I had both sets of details under 1 contact. Recently, Messages formed a "group" of me, and my wife's two phone numbers - not something we asked it to do. At that stage, messages would go to the wrong phone. I deleted all her work details and made a fresh contact, and yet there are still problems, with most of the personal messages associated with the work contact.