Need help with account separation

I have 2 iphones, phone (A) and phone (B) whenever i log into an account on either phones, the other will also log into it also,

example:

i log into account (1) with phone (A) and it will also log into account (1) in phone (B) without me doing anything

any ideas on how to seperate it so i can have account (1) on phone (A) and account (2) on phone (B) cause i wanna lend my phone to my friend, i tried logging out apple id and it still does the same thing