Some airlines offer such a product, but AY is not one of them. However, if you require an empty seat next to you for medical purposes or because of not being able to fit in one seat, then you can call them and make the booking over the phone, paying for an extra empty seat.

If you don't fall into that category, my suggestion would be to
1) check which plane type operates the flight, if it's E90, seating is 2+2 and there won't be any empty middle seats in J
2) if it's A320 fam., prereserve window and aisle, leaving the middle seat between you two empty and simply hope for the best. If that seat gets assigned to someone, they will be more than happy to move either to a window or an aisle seat once on board, giving you two seats next to each other