iPhones don't sync with other iPhones. The can each sync with iCloud, though. iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept just the same as the iCloud Photos Library. So, for instance, if you take a picture with iPhone #1, it is added to the iPhone #1's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your iPhone #2, then that picture is deleted at iCloud and on all the other devices.
If you don't want this to happen, then turn off iCloud Photos on the one you don't want to send and receive pictures from other devices. If you have no other devices, then you may not need iCloud Photos turned on with either phone.
It's unclear what you mean by "unsynched it."