New indoor grower

I have grown several times outside but this is my first indoor grow. I am using this light actually 2 of them. I think they were about $90 each on Amazon
1200W LED Grow Light, WAKYME Adjustable Full Spectrum Double Switch Plant Light with Veg and Bloom Button and Powerful Heat Dissipation System for Indoor Plants Veg and Flower(120Pcs LEDs)
I have 3 plants in 3 gallon bag that were started in March, and I have 5 in 2 gallon bags I started in April

It posted before I was done. Can I let both group grow 18 hours. Or will the plants tell me when they are ready to flower and then I adjust the lighting ?

Yes you can run them together and flower all at the same time..

Thanks, I moved some of the plants outside but left 4 inside..all 4 are flowering about 3-4 weeks in but I am noticing some discoloration of the leaves and stems. Lot of red stems and some yellowing and occasional red/brown leaf. I think it could be magnesium deficiency.
Especially since all 4 plants have red stems and some discoloration. I am attaching an images.


Some basic info. Running 2 led at 1200 w each 12 hours. Using fox farms trio, very sparingly. Average temp is about 61-64 degrees. Been this way since I have started
Plants look pretty healthy except for some leaves.