You could veg for years yet if a monster plant was your goal. Always great to know what sex the plants are before sticking them in a big finishing pot then find out some are male. I have 9 from seed just around 6 weeks but none of the tiny pre-flowers have proven sex yet. Soon I hope.
As long as you keep the light at 18/6 they'll stay in veg unless they are autoflowering plants. Those will flower under 24/0 light.
There is no rush to flower so you have lots of time to train them to ScroG if you want or you could flip to 12/12 right now if you are eager to get to harvest. Can always shove a plant pole in the pot and tie heavy colas to that if need be. Be prepared for them to double in height if no more but they look like they won't get outrageously tall in flower like a lean and lanky sativa plant will.
Do both plants have hairs? You say they are from seed so even if they are fem seeds there are occasional males.
It's very true that a larger plant will yield more. That's the same for any fruiting plant. Pot size makes a difference too. Bigger roots = bigger fruits is often said tho I use mainly 4gal pots max for my plants.
Those plant look pretty robust and likely to be heavy yielders even if flowered now so it's up to you.
I just noticed your light. Just a couple weeks ago I bought a lightly used 2x4x5'h tent with a smaller version of that light so I had a place to veg my babies while flowering a couple large plants in the main grow room. Mine is totally no name and claims to be 1500W but only pulls 207W from the wall. It is doing a great job vegging my babies so I like it and the price was right. I do my flowering under Hortilux Super HPS bulbs. Have those in 400, 600 and 1000W.
All from seed and a mix of boys and girls. 5 white pots are Nepali Temple and the 4 others are hi-CBD ones called Sebring's Revenge.
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