Adding secondary e-mail address in the "Primary Contact" is for record purposes only.
In this article:
For Firm/Agent's
Select "Gear wheel icon" > "Profile", and select "Contact Details" > "Additional email"

For Client's
Navigate to Gear Leaver > "Settings" > "Contact Details"

Enter via "Additional email".

Note that "Primary Email" is used on tax forms as a contact email and in clients emails for e-signature.