You unfortuantely can't. A mobile phone number is tied to one Xfinity ID / user.
On the email front, you can have a firstname.lastname@example.org as a workaround for using the same email with multiple identities.
Yes sorry Steve, that's the way the platforms are architected, as secondary users were designed for unique individuals within the household, not necessarily the same person with multiple email accounts. You could also get a Google Voice number and forward the SMS to your main number.