We asked a cast member beforehand and she said that it would be fine; Disney will always try to accommodate you. And considering BoG lunch is open seating, letting 2 people in instead of 1 isn't a big deal. And the crowd level was 9 that day, the reservations were all full. Now if it were a reservation for 1 and there were 5 of you then no I wouldn't expect them to let you in. But it's just 2 of you. They will let you in with a reservation for 1 and I will guarantee you this.