The reservation system does not care how you will be paying for dinners. It just takes a reservation. At the time you arrive at the restaurant, you tell the hostess how you are paying: Latitudes Voucher, Specialty Dining Package (that comes with FAS), Specialty Dining package (you buy on your own like the 2 dinner upgrade), other vouchers, cash (cabin charge).
130 days prior to departure, you can get online and book as many restaurants as often as you want. We did this on our last cruise. (full disclosure, there had been a bug in the the system that would not let you book a restaurant more than once for about a year,,,, but that bug has been fixed in their software and we booked one restaurant twice with no issue).
Technically, you can not use your Latitudes Vouchers to at same restaurant twice.