It's probably easier for most people to not think of them as two remaining doors, but two remaining sets. Originally, with one hundred doors, if the goal object is only behind one of them, then there would be a 1/100 probability it would be behind the initially chosen door, which comprises one set, while there's a 99/100 probability that the goal object is behind one of the doors in the set of not originally chosen doors. If 98/99 of the doors in the not originally chosen doors set are excluded as having the goal object, then this does not change that there's a 99/100 probability that the goal object is behind a door in this set, it just means it wasn't one of the other doors in the set.