Q: Iphones on different accounts won't unsync

i have two iphones. One is fromwork and the other is a personal account, so they are billed separately. The personal account is used by someone else. I don't have physical access to that phone. Somehow, the phones are stncing everything-history, favorites, etc. and I eould like to turn that feature off. How can I do that? 

I tried taking the personal phone off icloud and disabling sharing but it won't stop.