Used mixed occupancies based on both occupancies just like the book says...
403.1.1
To determine the occupant load of each sex, the total occupant load shall be divided in half. To determine the required number of fixtures, the fixture ratio or ratios for each fixture type shall be applied to the occupant load of each sex in accordance with
Table 403.1. Fractional numbers resulting from applying the fixture ratios of
Table 403.1 shall be rounded up to the next whole number.
For calculations involving multiple occupancies, such fractional numbers for each occupancy shall first be summed and then rounded up to the next whole number.