At what point do you turn the bloom section on? When the plant shows its sex? About 5 weeks into the grow? And when does counting begin, the day the seeds breaks the soil.
In short, there is no right or wrong. It sounds like you should activate both switches as you seem to have more than one plant in your grow space. The area covered with adequate intensity is smaller when only one switch is activated. As your plants are five weeks old, they should handle 300 PPFD well, and to achieve 300 PPFD over a larger area, you should activate both buttons.
We just made a detailed reply here on a similar question. I hope this information helps. Let me know if there is any confusion or if you have any other questions.