What happens if you are using same Apple ID on two phones

Assuming that you are not using a work Apple ID, there is no reason why you shouldn’t use the same Apple ID on both phones as long as both phones are yours. If someone is using your own personal Apple ID on another phone, that is not recommended. Everyone should have their own Apple ID.

If none of those things apply to you, you are fine. You should be using the same Apple ID on all of your devices.

Nothing. Many users have more than one device.