I've done this before with my wife's phone and mine. First, make a good backup of both phones using iTunes or a third party app such as iMazing 2. (If you have health info, encrypt the backups) Then turn off "Find My iPhone" on both phones. Next, restore the backups from each phone to the other. You may also have to rename the phones: "Wife's XR", "My 8+". That should do it. The backups are very important and it might be a good idea to backup to iCloud in addition to iTunes.
Someone else may have a better and easier way to do what you want, but what I outlined above worked for me.
EDIT: I almost forgot to mention this... if you and the wife wish to keep the same telephone numbers, you will need to swap the SIM cards. Without doing that, you'll be on the wrong number.