If both are connected to the same iCloud ID, one of them should be the new iPhone.
Change the name of the old phone, just for a bit, to be sure you remove the right iPhone from iCloud. The name of the iPhone can be edited in Settings - General - About - Name.
Where did you try to turn off Find my iPhone? It's easier in Settings - iCloud - Find my iPhone. All your iPhone will do, is ask for the iCloud ID's password. It won't offer two options. After entering the password, Find my iPhone will be turned off.