Google "extra seat jetblue" -
https://www.jetblue.com/flying-with-...ng-extra-seats
You cannot bring an additional personal item / carry-on, as that is per-person, not per-seat (regulated by FAA). As the site says for adding to an existing reservation:
An extra seat cannot be added to an existing booking, but if the seats are available, you can make a new booking. Please note you should contact us after making the booking or to make the booking so your original and new reservations can be linked, and won’t be mistaken as a duplicate booking and canceled.