I bought a few 20 gallon grow pots (20" diameter x 15.5" high), and they look quite big for one plant.
Do you think I can put two plants side by side? That'd be 10 gallons per plant. The only issue is they will end up pretty close together.
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I'd put exactly ONE in a 20" diameter pot. Otherwise you are going to crowd them up terribly.
"Square foot" gardening recommends 1 tomato plant per 2 square foot spacing - you grid off the planting area into 1' squares and for a tomato it would take 4 of them.
Regardless of whether this is a "determinate" (bush type) or "indeterminate" (vine type) tomato, trying to put two of them in the same pot is likely to make it a jumbled mess. Ultimately, you want some space for these to grow.
Regarding spacing of tomatoes in general, this year I spaced my tomatoes right at 3' apart and spaced the rows at 4' apart. I use some simple cages to contain my plants - mostly indeterminate types - and those are about 2' in diameter. So there is approximately 1' between the cages in the rows and approximately 2' of potential walking space in the rows. Of course, the tomatoes extend out from the cages and so there's a bit less than that and sometimes I have to move vines to scoot around the plants.
My point in all of this is that tomatoes are plants that need space and will benefit from the space. Having room allows air circulation and reduces the likelihood of non-insect problems.
Determinate varieties "behave" better but still perform better with space. I'll be honest with you - I had to learn this the hard way after cramming way too many plants in a given space in an attempt to "maximize" my yields. What I ended up doing most of the time was increasing my problems.