Just take clones of each plant, let them root, and then flower the clones. You will have your sex questions answered in about three weeks total. Switching to flower for two weeks to find the sex and then back to veg will cause your plants to "reveg" where the growth will pretty much stop for 4-6 weeks. But if you cut some clones, root them (usually about a week-10 days) and then flower them for 2 weeks once they are rooted, you will know the sex in only 3 weeks. Plus as KilllerWeed said you are risking turning your females into herms when you stress them like that. The light cycle of the flowering plant is delicate and shouldn't be messed with. Also in my experience, all the plants I grew from seed all showed preflowers in about 4-5 weeks, so the sex was already apparent without any change in the light schedule.