It is my understanding that each cabin can have only one reservation for each type of Palo serving made in ADVANCE. By that, I mean one dinner, one brunch, one tea. You can sometimes get a second ressie after you board.
The only way that I know to get more than one prior to booking is to have 2 cabin numbers and have each cabin make a ressie.
Not all of the ressies are placed on the internet. Some are held back for the concierge staff--if concierge level guests don't fill all of these, they are then released to the rest of us. It has been reported (but I can't confirm) that some additional ressies are held back till after boarding in case some people didn't know about making internet ressies or whatever.