Besides competeing for water/nutrients & light, another reason for NOT having 2 plants from seed in the same container:
let's say 1 turns out male, the other female.
So when the male grows balls you can't uproot it; that would kill the girl. So you'd have to cut it off at ground level. That means dead rotting roots from the cut off male intertwined with the roots of the female, not a good thing.