I believe, for the onboard booking perks to apply, at least one person on the reservation(s) must be on the current cruise (the one the reservation is made on). As in, if you have two rooms, at least one of the people on those two reservations must be on the current cruise. But, I'm not real sure about that.
But, yes, both rooms need to be on the same future cruise for both reservations to get the OBB perks, if they book two rooms.