They would compete for root space, nutrients, water, growing space, light.... basically everything. Have tried it many years ago and ended up with 2 very tired and small plants and they weren't even next to each other, but approx 6 inches apart. They both struggled with fighting each other for room and ended up with about one quarter the harvest as two plants of the same strains that wee grown individually. The two that I kept together were so nice that I didn't want to chop either one down in order to end up with only one seedling in the pot, but I later very much regretted that I didn't, or that I didn't try to separate them when they were still small. I thought I would end up with a double harvest, but due to the competition for root space, light, nutrients etc, I ended up with a half harvest. I would suggest not to try it unless your have space, time, room and are not interested in a big harvest and only do it out of curiosity or as an experiment. Basically, due to competition you will end up with 2 very mediocre plants rather than two normal plants. However, growing outdoors directly in the ground, it can work and I have planted two seedlings very close to each other and tied them down in opposite directions and this worked out ok because root growth was not restricted as much, but water requirements went way up and it was not really ideal. I now would not choose to plant them in the ground any closer than 3 feet minimum, if at all possible, Hope this helps,..... Organoman.