It seems to me that the problem is that you currently have two Apple ID's in use
One on the iPad and one on the iPhone.
And when you try to change the email you are not changing the apple ID but are trying to change the email on the apple ID. (and that email is already in use on the other account)
You need to sign out of one of the devices with the "spare" apple ID, and sign in with your Main apple ID.
You need to decide witch one of your two apple ID's should be the main one. Generally I would advice to check for witch one you have purchases on.
View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support
If the apple ID you want to keep has the wrong email adress, you would first need to free up the email adress(remove it from the other apple ID) and then wait 30 days before you can add it to your now main apple ID