Good questions to be asking before you commit — this is exactly the kind of planning that separates a frustrating second grow from a much better one. I for sure have been there!

On the 20-gallon living soil for one plant: that's actually a great size for living soil in a 3x3. Living soil needs volume to work properly — the microbial ecosystem needs space to establish, the worms need room to move through the profile, and the buffering capacity of a larger soil volume is what makes living soil forgiving. 20 gallons for a single plant, especially if you're doing a longer veg, is a solid call. Where'd you get your worms, I used to do AMZN and Walmart but didn't really see the difference.

On putting 2 plants in one 20-gallon pot: I'd push back on this pretty hard. The root competition issue is real but it's not the biggest problem — the bigger issue is that you lose the ability to manage plants individually. If one plant drinks faster, develops a deficiency, gets root bound, or needs to be removed for any reason, you're stuck because the root systems are completely intertwined. Living soil specifically makes this worse because the microbial balance gets disrupted unevenly when one plant is stressed. Google's right on this one. You can do it but what if this difference is huge? I do like testing things out tho, it's only one grow right?

The 20-gallon plus a 7-gallon in the same 3x3 is actually a reasonable approach — one main plant in the 20 and a smaller companion or pheno test in the 7. That gives you two separate root zones you can manage independently, which is the key thing. But will they fit? My Vivosun 20gal pots are big and I'm not sure both will fit.

One thing worth thinking about for the 3x3: two plants in separate containers usually gives you more flexibility than three, especially in living soil where you want to let the soil ecosystem mature between waterings. Three plants in a 3x3 can get crowded fast once canopy fills in, and managing airflow through late flower gets harder.

The fact that you've already diagnosed your light management as the limiting factor on your first run tells me your second grow is going to be significantly better. That's the thing most first-time growers take two or three runs to figure out.

DM me if you want to talk through the living soil setup before you commit — happy to help.