The free 2nd line is a "clone" of the 1st line. That means it shares the same phone number with the main line.

To have a 2nd phone number on the account (whether issued new or by porting one in), you would need to purchase a virtual number for $4.95/Mo or $36/Yr. That virtual number could then be assigned to the 2nd port if you like.

The account itself though only includes 1 phone number and that phone number and the 2nd line is simply a clone of the 1st and both lines share the same single phone number.