How do I unsync two iPhones that use the same Apple ID

On one of the phones go to Settings/[your name], scroll to the bottom and tap Sign Out. You will need your password to do this. You will be asked if you want to keep the content on the phone; that is up to you. Then sign in with a different Apple ID or create one if you don’t have one. The two phones will now have separate identities.

What are you trying to “unsync”? Two devices signed into the same Apple ID are going to have identical iCloud content on them. All I can think of is to go into the iCloud settings on one of the phones and turn off iCloud on all the apps using it.