How do I stop a different iphones information loading onto my iphone when a sync takes place?

Everytime I update my phone or my husbands phone our phone information is wrongly loaded onto each others phone. For example, I've just updated my phone to the latest software, my husbands Apple ID is now showing as my id on my phone - how do we stop this from happening? I'm sure there is a very simple and easy solution to this but I just don't know what it is. Please help 😕