Try asking your agent or X Rep. When we did book early or late dining the tables were assigned prior to boarding; a card was in our room with the table #. Then we'd go to the MDR and check the table location as soon as possible. If we thought we'd prefer another two top, we'd ask the maitre d' what was available and could we be relocated. I think we were asked at the time of booking did we prefer table for two or open table for 4, 6, or 8 as we were always assigned a two top. One time we asked our rep to request a two top under the MDR wine tower (S Class) which has those tables spread out better and more private...I think there's only 4 of them. Request was honoured.
Now we book Anytime Dining and yes, those tables are given as you arrive to the dining room; generally that's your table for the cruise if you dine approx. at the same time each night. We only ask to be relocated if the table is not suitable; Some would arrive just before the dining room opens and ask for a window two top. And yes all 2 tops are very close together in most cases.