2 iPhones Sharing One eMail Address

So I have run into trouble as you can see.

This is me entering the various fields in the Mail settings on my wife's phone.

I am using my name, my iCloud e-mail settings and I still get this dialog box so I am stymied.

So I tap on Accounts in Mail on my wife's iPhone and the next window that shows up lists the accounts -

iCloud

xxxxxx@northwestel.net (this is the primary address I use on my iMac and iPhone)

Add Account

Just to reiterate, the address that I am trying to add to her phone is -

xxxxxx@mac.com

since this in address in the cloud, I assume that I have to select iCloud from my 3 options above

When I do that, this is the window that comes up -

so I then turn on Mail and I am then asked to create a free

@icloud.com e-mail address to turn on iCloud Mail. So I then enter the xxxxxx@mac.com which is my address that I want functioning on her phone and I am greeted by the following dialog box <<this e-mail address is already taken. Try picking a new one>>. Well yes I know that it's already taken but I thought that following these steps would enable my address to work on her phone. Clearly this isn't the case. Where did I go wrong?

Will do. Thanks