How do I stop my iPhone and iPad from syncing with each other

That may be tough but on both devices you need to disable all Apple services.

start by going to settings then iCloud and logging out. Then go,through the entire list and anywhere it shows an account such as iTunes, mail, etc. sign out or delete anything apple related.

Cant figure out why you would want to do that but everyone has their reasons I suppose.

Tthe quickest way would be to reset the phone to factory. just sign out of iCloud then reset the whole device to factory in general, reset, erase all content and settings. Maybe not ideal but you're ensured that device won't sync at all. sign out of iCloud first to deactivate the activation lock Is why I mentioned that before the reset process As you would need your Apple ID again pond the forced reboot.

good luck