Link two accounts with same email

My father is elderly and having issues with paying bills, etc.  I am trying to help ease the pain with managing some accounts.  Bought him a cheap flip phone to keep in contact with me, on the advice of the sales rep, this flip phone had to be on a separate account, no big deal.  Now trying to link the two accounts so I can manage paying his bill.  On phone for 90 minutes with c.s., finally they gave up and sent me to tech support.  Tech support says you cannot link two separate account using single email address, makes no sense to me since i do this routinely with my investment accounts, credit cards, etc for wife and family, typically very simple process.  I don’t know if I am getting bad advice or this is true.  Would make a lot of sense for this to work so we can help manage bill payments of our elderly parents.  ANY ADVICE would be appreciated.