primary email id

primary email id

I'm trying to change my Primary Email Id.
I go to the appropriate tab in Settings, click the Edit button & enter the new email address & my password, but the existing email address doesn't change.
So I try to Add another Login Email Id, and when I enter the details get a message that the Email ID is already registered.
I have 2 personal zohomail accounts, & the email I'm trying to add is used as the Primary email on the other account.  Can't I use the same backup  email address for both my zoho accounts? as this would be the simplest solution - I don't want to have to remember which backup email address I used for which of my Zoho accounts, if I ever need to use it.