iPhone 7 and iPhone 8 linked. How do you make them independent from each other?

My wife has an iPhone7 and mine is an iPhone 8. When she calls or texts me it shows her phone with my name. I went into settings and changed her phone to her name and DOB and my phone then had her name and DOB. I changed it back to my name and DIOB on my phone and it went back to my name and DOB on her phone. If I download an app it usually appears on her phone. Somehow these phones are linked and whatever is done on one shows up on the other one. How do you unlink these phones so they are independent from each other?