If a room is used for two or more purposes, each of which is subject to different ventilating requirements, and it is possible to determine the space which will be used for each purpose, then each such space may be considered as a separate room and ventilated in accordance with the table of ventilating requirements applicable to that room. If it is not possible to determine the space which will be used for each purpose, the most restrictive, applicable ventilating requirements shall apply to the entire room.