The addition of a new line on your account can be used for supplemental services (ie Tablet or data-only plan) or another phone line. The services on the additional line would be separate from your current line. The additional phone line would have its own phone number and need to have its own plan. Fido currently does not have any shared plan offerings so each phone line's plan would need to include its own data allotment, if desired.
Hope this helps 😀