I grew some White Widow outside in a 70 qt tote 8 or 9 years ago. I placed one on the right side of the tote and I planted an Aurora Indica on the left side. The tote measured like 36" x18" x 18".
The white widow grew amazingly well but the poor AI was soon shadowed by the WW. It flowered but I only got like 2 1/2 ounces from it.
That was roughly an 18 gallon pot.
I drilled 20 or 30 holes in the bottom of the tote and dug a hole in the ground to put the tote in so the plant was at ground level.
That same year I used a garbage can to do the same thing. I dug a 3 ft deep hole to put the garbage can in. I did this so if I had to move them, I could. (rippers)
But That was the only time I grew two plants in the same container. One outgrew the other and took over basically.