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.